Fitch Ratings downgraded multiple Citigroup Mortgage RMBS series from triple-A to junk today after placing them on negative rating watch. The ratings agency made the downgrades as part of an ongoing review of prime and Alt-A RMBS transactions as the housing downturn continues to unwind. Many of the vintage 07 series involved in Citigroup’s RMBS downgrades migrated to double-C from triple-A but several made the leap to triple-C from triple-A. The agency also slashed five series of Bear Stearns RMBS from triple-A to double-C, and one from triple-A to triple-C. Bank of America Funding’s RMBS on the most part maintained its triple-A rating, although a single series previously placed on negative ratings plunged from triple-A to triple-C. Fitch recently revised its surveillance methodology for prime and Alt-A residential mortgage-backed securities to incorporate ResiLogic’s mortgage loss and default model, which determines a base-case loss expectation in conjunction with a transaction specific assessment of the pool’s actual performance. 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.