One pair of male rhesus monkeys reared in the laboratory with exclusive social access to each other during their second year of life were observed to display reciprocal mounting with anal penetration on several occasions. Their mounts were of comparable duration to those which have been observed in matching heterosexual pairs, by contrast with the brief mounts which are often observed in dominance interactions. The postures exhibited by the mounted male resembled those typical of females during heterosexual coitus, while the mounting male displayed the species-typical male copulatory position. Each male at times assumed either role. Ejaculation was not documented during these male-male interactions, although it may have occurred. Both males performed adequately with familiar and unfamiliar females (including ejaculation), but a preference test demonstrated that these males preferred contact with one another to contact with a sexually receptive unfamiliar female. These males displayed an unusual amount of affectionate behavior (by contrast with other rhesus males) including extended periods of contact with embracing and grooming. It is suggested that the relationship between these males was primarily based on mutual affection, although a sexual component was unmistakably present.