July 30 (Bloomberg) -- U.S. foreclosures rose 58 percent in the first half of 2007 from a year earlier, led by California and Florida, as more homeowners fell behind on their monthly mortgage payments, RealtyTrac Inc. said.

Lenders sent notices of default, scheduled auctions or repossessions to 573,397 properties in the January to June period, the Irvine, California-based seller of foreclosure data said in a report today. California foreclosures surged 170 percent to 104,572, the highest in the nation, and Florida gained 77 percent to 64,250.

Homeowners with adjustable-rate loans who are seeing their payments jump are finding it harder to avoid foreclosure by selling or refinancing their properties as the U.S. housing slump worsens. The inventory of homes for sale, measured in the time it would take to sell them all, was 8.8 months in June, matching the prior month as the highest since 1992, the National Association of Realtors said last week.

``Foreclosure activity shows no sign of slowing down,'' James Saccacio, RealtyTrac's chief executive, said in the report.

The results today reflect a new methodology by RealtyTrac. The company for the first time reported on what it calls ``unique addresses,'' or properties that have had at least one foreclosure- related legal filing. In previous reports, RealtyTrac had only reported the number of legal filings, which could have resulted in properties being double and triple counted.

To contact the reporter on this story: Kathleen M. Howley in Boston at kmhowley@bloomberg.net.

Last Updated: July 30, 2007 09:28 EDT