This paper describes the prevalence of anal sex among heterosexual adults in California and investigates the relationship of anal sex to other risk behaviors associated with AIDS and STDs. The study consisted of telephone interviews with a household probability sample of 3,545 California adults undersampling those age 44 and older. Seven percent of the sexually active respondents, 8% of males, and 6% of females reported having anal sex at least once a month during the year prior to the survey. Of these, most engage in this activity one to five times per month, and about 60% report never using condoms. Younger respondents and those who were not married were more likely to report anal intercourse. Respondents who had anal sex were more likely to report standard AIDS risks and lifestyle risks associated with STDs, and to engage in recreational use of drugs and alcohol. Both anal sex and condom use during anal intercourse were poorly predicted by these demographic and risk variables. It is concluded that a non-trivial proportion of California heterosexual adults engages in anal sex regularly, most without condoms, and those who have anal sex are more likely to have other risk behaviors associated with AIDS and STDs. These results suggest that anal sex must be addressed specifically in clinical and educational programs designed to reduce the spread of AIDS.