By NICK TIMIRAOS

New data suggest that foreclosures are rising in more expensive housing markets.

About 30% of foreclosures in June involved homes in the top third of local housing values, up from 16% when the foreclosure crisis began three years ago, according to new data from real-estate Web site Zillow.com. The bottom one-third of housing markets, by home value, now account for 35% of foreclosures, down from 55% in 2006.

The report shows that foreclosures, after declining earlier this year, began to accelerate in the late spring and that more expensive homes have more recently accounted for a growing share of all foreclosures. "The slope of that curve in recent months is much sharper than it was recently," said Stan Humphries, chief economist for Zillow. Rising foreclosures among more-expensive homes could create added pressure for a housing market that has shown signs of stabilizing in recent months as sales of lower-priced homes pick up.

The Zillow research compared homes against the median values for their local market and broke each market into three tiers by value. Zillow then looked at the share of monthly foreclosures in each tier over the past decade.

Foreclosures are rising in more expensive markets as home values in those areas fall, leaving more homeowners with mortgages that exceed the value of their properties. Prime loans accounted for 58% of foreclosure starts in the second quarter, up from 44% last year, according to the Mortgage Bankers Association. Subprime mortgages accounted for one-third of foreclosure starts, down from one-half last year.

The prime category includes so-called exotic mortgages that were increasingly used to buy more expensive homes, including interest-only mortgages that allowed borrowers to defer principal payments during an initial period. Borrowers often aren't able to refinance out of these products because the drop in home values has left them with little equity in their homes.

Default rates are particularly high and expected to rise on option adjustable-rate mortgages, which allow borrowers to make minimum payments that may not cover the interest due. Monthly payments can increase to sharply higher levels after five years or when the outstanding balance reaches a certain level. A study by Fitch Ratings found that 46% of option ARMs were 30 days past due last month, even though just 12% of such loans have reset to higher monthly payments.

Zillow estimated that nearly one in four homes with mortgages was worth less than the value of the property at the end of June. Mr. Humphries said he didn't expect to see foreclosure volumes level off until later in 2010.

Write to Nick Timiraos at nick.timiraos@wsj.com