Homosexual activity among adult females in a heterosexual group of rhesus monkeys (Macaca mulatta) is documented and discussed. The mounter was usually in the follicular stage of the menstrual cycle and the mountee was in the ovulatory period. Activity during the luteal phase was low for all animals. Female mounting did not appear to be a preparation or substitute for heterosexual activity. Ritualized solicitation patterns among the females were intense and varied, strongly resembling those for heterosexual pairs. Dominance, partner preference, sexual stimulation, reproductive state, individual characteristics, and past experience are discussed in the framework of multiple factors influencing the homosexual relationship.