Persons living with HIV (PLH) often attribute HIV status to sexual partners based on observable partner characteristics. The present study investigated the relationship of sexual behavior with most recent "main" partner to that partner's perceived serostatus among 1,232 PLH interviewed in clinics and community agencies in Los Angeles, California. PLH who believed their most recent main partner to be HIV-negative more often identified partner appearance as a basis for their perceptions than those who believed their most recent main partner to be HIV-positive. PLH who perceived their most recent main partner as HIV-negative were more likely to assume responsibility for partner protection and always to use condoms, and less likely to report recent unprotected vaginal or anal sex with that partner. Unprotected receptive anal intercourse with their most recent main partner was less common among African American, Latino, and White participants who believed that partner to be HIV-negative. Although PLH appear protective toward HIV-negative main partners, interventions to encourage valid methods of identifying partner serostatus are needed.