Moody's Investors Service on Wednesday downgraded 37 tranches of '07-vintage residential mortgage-backed securities (RMBS) issued by Credit Suisse and worth a combined $761m. Increasing delinquencies, slower prepayments and mounting losses in underlying collateral drove the downgrades, Moody's said in a statement. The rating agency recently adjusted its loss expectation on ALt-A pools as a result of continuing deterioration of the housing market, and many loans backing the collateral in the Credit Suisse transactions are Alt-A mortgages. Of 31 "Aaa" tranches, Moody's dropped three to "Aa3" and one to "A1," and slashed one to "Baa3," two to "Ba2," 20 to "B3" and four to "Caa1." All of these were previously placed under review for possible downgrade in October except for one of the tranches slashed to "B3," which had only been assigned its "Aaa" rating in March 2007. The other six tranches involved in the actions slipped from "Aa1"; Moody's downgraded one to "B3," one to "Caa1" and four to "Ca." All six of these had been placed under review for possible downgrade in October. The rating agency said it would continue its ongoing review of the tranches' expected losses as the performance of underlying Alt-A mortgages changes. Write to Diana Golobay. Disclosure: The author held no relevant investment positions when this story was published. Indirect holdings may exist via mutual fund investments.