Two hypotheses were tested in this study: (a) moral reasoning and risky sexual behaviors are inversely correlated; (b) the relationship between AIDS knowledge and sexual behavior is mediated by moral reasoning such that AIDS knowledge and risky sexual behaviors are inversely correlated for higher-level moral reasoners but not for lower-level reasoners. Subjects were 103 undergraduate students who completed an instrument assessing moral reasoning, the Defining Issues Test, and two questionnaires assessing sexual behavior and knowledge about AIDS. Factor analysis identified five sexual risk-taking factors with alphas of .60 or higher. Results supported both hypotheses: (a) Risk taking during sexual intercourse (i.e., lower likelihood of using condoms) and risk taking during anal sex were significantly inversely correlated with moral reasoning; (b) For high moral reasoners, two measures of sexual risk taking were significantly negatively correlated with knowledge about AIDS: as knowledge increased, risk taking during sexual intercourse and engagement in a variety of sexual experiences decreased. The reverse relationship was true for low moral reasoners. Implications for moral development theory and research as well as for AIDS prevention campaigns are discussed.