A common finding in studies of heterosexual young adults is that condom use is more likely with casual partners than with regular partners. However, few studies have examined whether correlates of condom use differ with different partners. In this study, 103 heterosexual men and women completed a condom use diary for a period of up to 6 months. Condom use during sexual encounters with regular partners was mainly determined by established patterns of behavior. In contrast, condom use with casual partners was determined by the interaction between the sexual partners during the encounter, and was not influenced by the attitudes and beliefs of the individual. These differences have implications for interventions. They suggest that condom use with casual partners may be increased by furnishing young adults with skills and confidence for negotiating condom use. For young adults in regular relationships, we must ensure that routine patterns of condom use are appropriate for the levels of risk to which the partners are exposed.