The Internal Revenue Service has ruled that same-sex couples must be treated the same as heterosexual couples under a feature of California tax law. Advocates for the change say it is the first time the agency has acknowledged gay couples as a unit for tax purposes.

The change reverses a 2006 IRS ruling and opens a tax benefit to many same-sex couples that wasn't available before. It may affect couples in Nevada and Washington state, as well.

Specifically, the agency said nearly 58,000 couples who are registered as domestic partners in California must combine their income and each report half of it on their separate tax returns. Same-sex couples account for an estimated 95% of the state's domestic partnerships; partnership status is also available to heterosexual couples in which one partner is over age 62.

"For the first time ever, I'm able to file federal taxes that, in a small way, acknowledges what's going on in my relationship," said Eric Rey of Berkeley, Calif. Mr. Rey and his partner requested the IRS ruling, first during the Bush administration and again this year.

The pair wed during a brief window when same-sex marriage was legal in California, but the issue arises because the men are registered as domestic partners. Same-sex couples, even if they are legally married in their home states, may not file joint federal tax returns. The federal Defense of Marriage Act, passed in 1996, defines marriage as between one man and one woman and bars federal agencies from interpreting it otherwise.