In a study of risk behavior among heterosexuals, 9% of a national sample and 12% of a sample drawn from high-risk cities report having two or more sexual partners in the year before the survey. In both samples, sex with multiple partners is most common among men, younger people and the unmarried. Multivariate analyses show that racial and ethnic differences in the proportion of respondents with multiple partners vary by marital status. The results indicate that many heterosexuals are failing to protect themselves against sexually transmitted diseases: Among respondents with multiple partners, only 18% of men and 22% of women always use condoms with their primary partner, and 28% of men and 32% of women always use them with secondary partners. These proportions do not increase significantly with the number of partners; in general, almost half of men and women with multiple partners never use condoms.