If humans faced recurrently over evolutionary history the adaptive problem of competition with same-sex friends for mates, they may have evolved psychological mechanisms designed to prevent and combat mating rivalry with same-sex friends. Four studies were conducted to test hypotheses about the design of these mechanisms. In Studies 1 and 2 (N= 406 and N= 342, respectively) we found that, as predicted, people experience more upset in response to imagined rivalry from a friend than from a stranger. In Study 3 (N= 455), in which a between-subjects design was utilized, we found that women's, but not men's, willingness to become friends with a member of the same sex is lower when the person is described as sexually promiscuous. In Study 4 (N= 169) we found that people report being deceived by friends about mating rivalry more often than they themselves report engaging in deceit about rivalry, and women more than men deceive each other about how sexually experienced and promiscuous they are. Discussion addresses implications of the findings and the use of an evolutionary approach for understanding conflict in same-sex friendship.