Moody's Investors Service lowered its ratings on numerous residential mortgage backed securities (RMBS), as the agency continues to adjust ratings to reflect its updated criteria on mortgage pools issued in 2005 to 2007. The New York-based ratings service took these steps due to "continued performance deterioration in subprime pools in conjunction with home price and unemployment conditions that remain under duress." Most of the collateral backing the deals consists of first-lien, fixed or adjustable-rate subprime residential mortgages. Moody's downgraded 63 subprime RMBS worth $5.91bn issued by Option One in 13 transactions. Ratings on seven RMBS transactions worth $2.3bn issued by Fieldstone were affected with 26 downgrades, one upgrade and the confirmation of five ratings. The ratings on $2.3bn of Alt-A RMBS pools issued by MASTR Alternative Loan Trust and MASTR Adjustable Rate Mortgages Trust were changed, including 72 downgrades and three upgrades. Three ratings were confirmed. Two subprime RMBS valued at $50.7m issued by RASC Series 2002-KS6 Trust were downgraded. The loan pool consists of one pool that includes fixed-rate mortgage loans and a second that includes adjustable-rate loans. Analysts cited losses in both groups and the balance of loans delinquent 60 days or more, including loans in foreclosure and real estate owned (REO), compared to the credit enhancement provided by subordination and excess spread in the downgrades. The pools remain on review for another possible downgrade as Moody's completes its review. Moody's updated its ratings on $213m of subprime RMBS issued by IXIS through in three transactions. Analysts confirmed the ratings of two, upgraded the ratings of two others, and downgraded nine. Write to Jason Philyaw.