A new and understudied HIV epidemic is quickly unfolding in the Central and Eastern European countries of the former Soviet Union. Men who have sex with men (MSM) in Russia constitute a population highly vulnerable to sexually-transmitted HIV infection. In a community sample of 434 Russian MSM accessed in gay venues in St. Petersburg, 126 had had both male and female partners in the past three months. In this paper, we report on their risk characteristics. Forty-five per cent of men reported recently engaging in unprotected anal intercourse with their male partners. Respondents had a mean of 3.3 male and 3.4 female partners in the past three months, and most had multiple male and female partners in this time period. There were serious and significant gaps in the AIDS risk knowledge levels of these men, and most believed they had no personal contact with HIV-positive people. Bisexual men were more likely than exclusively gay men to have engaged in commercial sex and tended to have lower AIDS risk knowledge. Although they did not differ in average age, bisexual compared to gay men more recently had their first sex with a man. Multivariate logistic and linear regression analyses showed that condom and safer sex attitudes, perceived norms, AIDS risk knowledge and age at first sex with a man were independent predictors of high-risk behaviour among bisexual men. HIV prevention interventions for bisexual men should address their sexual practices with both male and female partners, correct misconceptions about risk, address behaviour practices rather than gay identity, and recognize risk issues faced by the female partners of bisexual men.