NEW YORK (Bloomberg) - Credit rankings on more than 37,500 bonds, most of them municipal, have been lowered or adjusted following Fitch Ratings' cut last week of its assessment of insurer XL Capital Assurance Inc., the company said on Tuesday.

XL Capital Assurance is a subsidiary of Berrmuda-based Security Capital Assurance Ltd., of which XL Capital Ltd. owns 46 percent.

About 3,200 XL-insured bonds have underlying ratings from Fitch equal to or higher than the one provided by the insurer's corporate guarantee, which was cut to A from AAA on January 24 and put under watch for a further downgrade, according to a list posted on Fitch's Web site.

Fitch, the New York-based unit of Paris-based Fimalac SA., is the first of the three major rating companies to strip top grades from two bond insurers, including Ambac Assurance Corp., hurt by weakening values of non-municipal debt they backed.

Fitch last month said insured bonds would carry the higher of the guarantor's rating or the grade of the borrower, US states and local governments in many cases.

The company last week said it would withdraw its insured ratings on bonds that don't have a public underlying rating from Fitch, though do carry higher borrower assessments from competitors, Standard & Poor's or Moody's Investors Service.