SAN FRANCISCO, JUNE 23 -- Gay men are relapsing into unsafe sexual behavior, and other groups at high risk for infection by the virus that causes AIDS are ignoring warnings about the dangers of unprotected sex, according to researchers at the International Conference on AIDS here.

This behavior worries many AIDS experts who fear that the failure of "safe sex" education, the center of efforts to fight AIDS over the past decade, could exacerbate the already worsening epidemic.

It has also prompted sharp criticism of the approaches taken by federal and local health officials to promote safe sex, which some AIDS experts say have been too timid and too limited to change the behavior of high-risk groups or other groups such as sexually active heterosexual high school and college students.

"To stop AIDS, we are going to have to be more explicit. We are going to have to spend more money. And we are going to have to touch on issues that are culturally sensitive," said Jean McGuire, executive director of the AIDS Action Council.

The news from the studies is not entirely discouraging. Among gay men overall, for example, the percentage who practice safe sex has risen from 31 percent to 58 percent in the past four years, and high-risk behavior has dropped to less than 10 percent.

But behind that overall picture, researchers said, are several disturbing trends:

Gay and bisexual men under age 30 are twice as likely to engage in unprotected anal sex -- the sexual act most likely to transmit the AIDS-causing virus, HIV -- than older men.

Half of black gay men continue to engage in anal intercourse without a condom.

In smaller U.S. cities, where the full brunt of the AIDS epidemic has not yet hit, gay men practice unprotected sex at rates as much three times higher than those in larger urban areas such as New York and San Francisco.

19 percent of gay and bisexual men who practice safe sex report relapsing to unsafe behavior over a four-year period.

Sexually active, heterosexual high school and college students only rarely use condoms. A Canadian study found that only a quarter of college males always uses condoms. And of the 21 percent of males who report 10 or more sexual partners, only 20 percent reported regularly using condoms.

According to Mervyn Silverman, president of the American Federation for AIDS Research, the problems of continuing high-risk behavior among gay men "must be addressed if we hope to avoid a second, even larger wave of AIDS cases."

At the root of the problem, AIDS experts say, is that traditional safe-sex campaigns have been too conservative to be effective.

For example, officials at the Centers for Disease Control, which conducts the country's most extensive AIDS prevention campaign, describe their efforts primarily as educational -- telling people about the AIDS virus and giving the knowledge and skills to protect themselves from it.

But according to the results of a three-nation behavioral study presented here, simply being informed about the AIDS virus does not make people any more likely to practice safe sex.

Instead, the study found, only if a person knows someone who has contacted AIDS or is convinced that he or she is at high personal risk of contracting the disease does there apear to be an increase in condom use.

"Disembodied knowledge about AIDS is not effective," said James Wells, senior policy analyst at Project Hope in Washington, the health-care group that conducted the study. "You have to create a fear of the disease in people for whom that is a reasonable response."

Experts also stress the importance of making condoms appear sexually attractive, particularly because the target groups for AIDS prevention are considered sexually "positive," people who both enjoy and engage in sex a great deal.

"Safe sex is not like dieting," said Thomas Coates, co-director of the Center for AIDS Prevention Studies at the University of California at San Francisco. "It has deeper emotional meaning and the message has to be erotic and intimate."

The CDC has guidelines stating that prevention efforts must meet individual community standards, a stipulation that even CDC officials concede makes explicit safe sex messages -- particularly in schools -- difficult to implement.

Even an ideal prevention program, however, can only go so far in changing behavior. One major reason given by gay men for relapsing to unsafe sex practices, for example, was because of the use of drugs or alcohol, a situation difficult to remedy through prior education.

Unsafe sexual behavior also is concentrated among the young, a group whose behavior is notoriously difficult to modify, researchers said.

And if fear of contracting AIDS and knowledge of those who have the disease is the key factor in promoting safe sex, how can a TV commercials and public health messages ever be truly effective? "This is not going to be easy," said Wells. "If we can't get people to stop smoking, how can we get them to stop having unprotected sex?"