The rate of U.S. home mortgages overdue or in foreclosure rose again in the second quarter as housing markets weakened, particularly in California and Florida, and more borrowers defaulted on so-called prime loans.

Among mortgages on one- to four-family homes, 9.16% were at least a month overdue or in the foreclosure process in the second quarter, according to the latest survey by the Mortgage Bankers Association, a trade group. That is up from 6.52% a year earlier and is the highest level since the MBA began such surveys 39 years ago.

The foreclosure crisis, generally considered the worst since the Great Depression of the 1930s, began in late 2006 with a surge in defaults on subprime loans, those to people with weak credit records. As falling real-estate prices have left more people owing more than the current value of their homes, defaults have gradually spread to include more prime loans. Among the most troubled prime loans are option adjustable-rate mortgages, which let borrowers start out with very low monthly payments and pose much bigger ones after a few years.

Option ARMs are "a driving factor" in the surge of prime loans going bad, said Jay Brinkman, the MBA's chief economist.

For prime loans, 5.35% of loans were past due or in foreclosure in the latest quarter. For subprime, the rate was about 30%.

In the latest quarter, 2.75% of all loans were in the foreclosure process, up from 1.40% a year earlier.

California and Florida account for about one in five mortgage loans outstanding, but 39% of loans that went into the foreclosure process in the quarter were in those two states. In both states, builders and speculators vastly overestimated demand for housing during the boom of the first half of this decade. The resulting glut of homes for sale has pushed down prices. Lower prices, in turn, make it much harder for borrowers to refinance if they fall behind on their payments. The drop in values also means people have less incentive to keep making payments.

Other states in which loans went into foreclosure at a rate faster than the national average during the second quarter were Nevada, Arizona, Michigan, Rhode Island, Indiana and Ohio.

Among loans insured by the Federal Housing Administration, 14.87% were overdue or in foreclosure, up from 14.73% a year earlier. The portion of FHA loans going bad is likely to increase in the quarters ahead because of a surge in new loans insured by the federal agency.

The share of new mortgages insured by the FHA leaped to 23% in July from a low of 1.8% in 2006, according to Inside Mortgage Finance, a trade publication. Guy Cecala, publisher of Inside Mortgage Finance, said the FHA's share might reach 30% by year end. The FHA is taking a far bigger share of the market because investors last year began shying away from buying mortgage securities that don't have backing from a federal agency or from government-sponsored mortgage investors Fannie Mae and Freddie Mac. Fannie and Freddie recently have become more cautious about buying or guaranteeing mortgages because default-related losses have depleted their capital.

Borrowers can get FHA-insured loans with down payments as small as about 3%.

Thomas Lawler, a housing economist in Leesburg, Va., said the pace of foreclosures is likely to continue increasing through at least early next year. The increases will come largely from mortgages that turned out to be "not very prime," he said. Mr. Lawler added that lenders "have become massively more aggressive" in cutting prices on foreclosed homes in an effort to reduce their inventories.

Write to James R. Hagerty at bob.hagerty@wsj.com

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