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Prime Master FundSchedule of Investments (Unaudited Holdings as of 04/30/2021)Presented below is the monthly portfolio holdings report for Master Trust - Prime Master Fund (the "Master Fund"). Each of the funds in the UBS Select Prime series is a "feeder fund" that investsin individual money market securities through the Master Fund.Weighted Average Maturity: 49 daysWeighted Average Life: 61 daysIssuerCategory of investmentABN AMRO Bank NVNon-Negotiable Time DepositANZ New Zealand International Ltd.CUSIPPrincipal amountMaturity dateFinal maturitydateCoupon or YieldValue% of 004.08Financial Company Commercial 4,4780.45ANZ New Zealand International Ltd.Financial Company Commercial 0,0000.25ANZ New Zealand International Ltd.Financial Company Commercial 3,5880.35ANZ New Zealand International Ltd.Financial Company Commercial 8,8360.35ASB Finance Ltd.Financial Company Commercial 9,0190.72ASB Finance Ltd.Financial Company Commercial 4,7080.71ASB Finance Ltd.Financial Company Commercial 9,2050.37ASB Finance Ltd.Financial Company Commercial 9,4600.68ASB Finance Ltd.Financial Company Commercial 8,4490.76Bank of Nova ScotiaFinancial Company Commercial 2,2700.28Barclays Bank PLC Tri Party RepoU.S. Government Agency .010200,000,0002.27Barton Capital SAAsset Backed Commercial 9690.06Bedford Row Funding Corp.Financial Company Commercial 0,0000.45Bedford Row Funding Corp.Financial Company Commercial 0,0000.48BNP Paribas SA Tri Party RepoOther Repurchase 0000.57BNP Paribas SA Tri Party RepoOther Repurchase 0000.34BNZ International Funding Ltd.Financial Company Commercial 6,8520.52BPCE SAFinancial Company Commercial 5,2000.57Caisse des Depots et ConsignationsNon-Financial Company 0.17624,968,3450.28Canadian Imperial Bank of CommerceFinancial Company Commercial 3,8190.90Canadian Imperial Bank of CommerceFinancial Company Commercial 0,0900.90Page 1 of 6

Canadian Imperial Bank of CommerceFinancial Company Commercial 0,6160.52Canadian Imperial Bank of CommerceFinancial Company Commercial 2,6520.42Chariot Funding LLCAsset Backed Commercial 2,4470.67Collateralized Commercial Paper FLEX Co. LLCFinancial Company Commercial 1420.07Collateralized Commercial Paper FLEX Co. LLCFinancial Company Commercial 2,5630.28Collateralized Commercial Paper FLEX Co. LLCFinancial Company Commercial 9,0640.40Collateralized Commercial Paper V Co. LLCFinancial Company Commercial 4,1160.90Collateralized Commercial Paper V Co. LLCFinancial Company Commercial 0,0790.15Cooperatieve CentraleCertificate of 019,1580.72Cooperatieve CentraleCertificate of 999,9980.57Cooperatieve Rabobank UACertificate of 009,2990.59Credit Agricole CIBNon-Negotiable Time 0001.34Credit Industriel et CommercialFinancial Company Commercial 7,2300.70DBS Bank Ltd.Financial Company Commercial 4,4220.58DBS Bank Ltd.Financial Company Commercial 6,4560.96DBS Bank Ltd.Financial Company Commercial 9,8750.28DBS Bank Ltd.Financial Company Commercial 1,2530.45Dexia Credit Local SAFinancial Company Commercial 9,5200.54Dexia Credit Local SAFinancial Company Commercial 4,2150.68Erste AbwicklungsanstaltNon-Financial Company 0.08549,995,8680.57Erste Finance LLCNon-Financial Company 10.150179,996,9992.04Erste Finance LLCNon-Financial Company 10.150114,997,1251.30Fairway Finance Co. LLCAsset Backed Commercial 1,7840.68Fairway Finance Co. LLCAsset Backed Commercial 0,0370.50Fairway Finance Co. LLCAsset Backed Commercial 5,1400.68Fairway Finance Co. LLCAsset Backed Commercial 8,8680.37Fairway Finance Co. LLCAsset Backed Commercial 3,2740.25Federation des Caisses Desjardins du QuebecFinancial Company Commercial 998,9002.49Federation des Caisses Desjardins du QuebecFinancial Company Commercial 999,1931.37J.P. Morgan Securities Tri Party RepoOther Repurchase 0000.85J.P. Morgan Securities Tri Party RepoOther Repurchase 0000.79J.P. Morgan Securities Tri Party RepoOther Repurchase 0000.85J.P. Morgan Securities Tri Party RepoU.S. Government Agency .010300,000,0003.40Page 2 of 6

J.P. Morgan Securities Tri Party RepoU.S. Government Agency .020200,000,0002.27Liberty Street Funding LLCLiberty Street Funding LLCAsset Backed Commercial 4,8170.40Asset Backed Commercial 9,5951.02Liberty Street Funding LLCAsset Backed Commercial 9,3790.28LMA Americas LLCAsset Backed Commercial 6,8170.71LMA Americas LLCAsset Backed Commercial 6,7500.57LMA Americas LLCAsset Backed Commercial 8350.07LMA Americas LLCAsset Backed Commercial 7,6020.24LMA Americas LLCAsset Backed Commercial 6,9860.34LMA Americas LLCAsset Backed Commercial 8,2660.48LMA Americas LLCAsset Backed Commercial 5,5550.21Merrill Lynch Pierce Fenner & Smith, Inc. Tri Party RepoOther Repurchase 0000.85Merrill Lynch Pierce Fenner & Smith, Inc. Tri Party RepoOther Repurchase 0,0002.66Merrill Lynch Pierce Fenner & Smith, Inc. Tri Party RepoOther Repurchase 0000.28Merrill Lynch Pierce Fenner & Smith, Inc. Tri Party RepoU.S. Government Agency .010200,000,0002.27Mitsubishi UFJ Trust & Banking Corp.Financial Company Commercial 9,5370.45Mitsubishi UFJ Trust & Banking Corp.Financial Company Commercial 1,2000.57Mitsubishi UFJ Trust & Banking Corp.Financial Company Commercial 0,2050.57Mitsubishi UFJ Trust & Banking Corp.Financial Company Commercial 9,5800.57Mitsubishi UFJ Trust & Banking Corp.Financial Company Commercial 2,1230.34Mizuho Bank Ltd.Financial Company Commercial 4,9780.73Mizuho Bank Ltd.Financial Company Commercial 990,7521.14Mizuho Bank Ltd.Financial Company Commercial 8,9080.45Mizuho Bank Ltd.Financial Company Commercial 9,1470.45Mizuho Bank Ltd.Non-Negotiable Time 00.96MUFG Bank Ltd.Certificate of 002,9560.51National Australia Bank Ltd.Financial Company Commercial 0,7770.79National Bank of CanadaFinancial Company Commercial 3,7420.88National Bank of CanadaFinancial Company Commercial 2,6750.50National Bank of CanadaFinancial Company Commercial 5,3720.57NatixisNon-Negotiable Time 0002.04Natixis SAFinancial Company Commercial 7,5400.70Nieuw Amsterdam Receivables Corp.Asset Backed Commercial 1,5000.91NRW BankFinancial Company Commercial 998,6932.66Page 3 of 6

NRW BankFinancial Company Commercial Paper62939LS59Old Line Funding LLCAsset Backed Commercial 7,0500.57Old Line Funding LLCAsset Backed Commercial 7,4870.51Old Line Funding LLCAsset Backed Commercial 8,7730.93Old Line Funding LLCAsset Backed Commercial 0,2220.57Old Line Funding LLCAsset Backed Commercial 9,0440.44Old Line Funding LLCAsset Backed Commercial 2,5990.34Oversea-Chinese Banking Corp. Ltd.Certificate of 011,6200.50Oversea-Chinese Banking Corp. Ltd.Financial Company Commercial 1,5780.71Oversea-Chinese Banking Corp. Ltd.Financial Company Commercial 0,5120.58Royal Bank of CanadaCertificate of 017,7450.36Skandinaviska Enskilda Banken ABCertificate of 003,8650.71Skandinaviska Enskilda Banken ABCertificate of 009,4020.71Skandinaviska Enskilda Banken ABFinancial Company Commercial 1,8360.49Skandinaviska Enskilda Banken ABFinancial Company Commercial 7,8070.51Skandinaviska Enskilda Banken ABFinancial Company Commercial 6,5700.51Societe Generale SAFinancial Company Commercial 9,9250.17Societe Generale SAFinancial Company Commercial 998,8301.33Starbird Funding Corp.Asset Backed Commercial 9,8190.33Starbird Funding Corp.Asset Backed Commercial 4,0500.23Sumitomo Mitsui Banking Corp.Certificate of 006,0130.71Sumitomo Mitsui Banking Corp.Certificate of 005,4710.61Sumitomo Mitsui Banking Corp.Certificate of 000,6360.57Sumitomo Mitsui Banking Corp.Financial Company Commercial 1,5200.23Sumitomo Mitsui Trust Bank Ltd.Certificate of 003,8910.75Sumitomo Mitsui Trust Bank Ltd.Certificate of 002,6790.45Sumitomo Mitsui Trust Bank Ltd.Certificate of 004,7720.57Sumitomo Mitsui Trust Bank Ltd.Financial Company Commercial 5,5000.91Sumitomo Mitsui Trust Bank Ltd.Financial Company Commercial 5,1460.34Svenska HandelsbankenCertificate of 005,6840.40Svenska HandelsbankenFinancial Company Commercial 7,3540.42Svenska HandelsbankenFinancial Company Commercial 6,8890.73Svenska HandelsbankenFinancial Company Commercial 9,5550.57Svenska HandelsbankenFinancial Company Commercial 0,4210.40Thunder Bay Funding LLCAsset Backed Commercial 6,0500.34Thunder Bay Funding LLCAsset Backed Commercial 2,9300.57Toronto-Dominion BankCertificate of 028,0380.96Page 4 of 6100,000,00005/05/2105/05/210.05099,999,3061.13

Toronto-Dominion BankFinancial Company Commercial 9,2001.02United Overseas Bank Ltd.Financial Company Commercial 6,3140.68United Overseas Bank Ltd.Financial Company Commercial 2,6040.74Versailles Commercial Paper LLCAsset Backed Commercial 0,0000.50Versailles Commercial Paper LLCAsset Backed Commercial 0,0000.49Victory Receivables Corp.Asset Backed Commercial 005,4151.47Westpac Banking Corp.Financial Company Commercial 9,9990.51Westpac Banking Corp.Financial Company Commercial 6,7870.51Westpac Securities NZ Ltd.Financial Company Commercial 0,0970.66Westpac Securities NZ Ltd.Financial Company Commercial 8,0090.528,823,178,000Total8,821,737,882100.00The master funds are not publicly offered; certain information is provided as required by US Securities and Exchange Commission regulations. Information regarding related feeder money marketfunds is available by contacting UBS Asset Management at 888-793-8637 or by visiting http://www.ubs.com/usmoneymarketfunds.Investors in the fund should read and carefully consider the investment objective, risks, charges and expenses before investing. The fund’s Registration Statement – Part A contains this and otherinformation about the fund. Contact the UBS Asset Management National Sales Desk at 888-793-8637 for a current Part A. An investment in the fund is only one component of a balancedinvestment plan.Investors should note that current and future portfolio holdings are subject to risk.You could lose money by investing in a money market fund. Because the price of interests in a money market master fund will fluctuate, when you sell your shares of the related feederfund, your shares of the related feeder fund may be worth more or less than what you originally paid for them. A money market master fund may impose a fee upon sale of your sharesof each related feeder fund or may temporarily suspend your ability to sell shares of each related feeder fund if a money market master fund's liquidity falls below required minimumsbecause of market conditions or other factors. An investment in a money market fund is not insured or guaranteed by the Federal Deposit Insurance Corporation ("FDIC") or any othergovernment agency. A money market fund's sponsor has no legal obligation to provide financial support to a money market fund, and you should not expect that the fund's sponsor willprovide financial support to a money market fund at any time.Not FDIC insured. May lose value. No bank guarantee.The fund is actively managed, and its holdings are subject to change and should not be considered a recommendation to buy or sell individual securities.UBS Asset Management (US) Inc. is the master fund’s placement agent.U.S. Treasury Repurchase Agreements are collateralized only by U.S. Treasuries (including Strips) and cash.U.S. Government Agency Repurchase Agreements are collateralized only by U.S. Government Agency securities, U.S. Treasuries, and/or cash.Page 5 of 6

Other Repurchase Agreements are collateralized by securities other than U.S. Treasuries, U.S. Government Agency securities and cash.*% of TotalRepresents the percentage of the total principal amount of the securities in the portfolio.Page 6 of 6