A type C retrovirus was isolated from a continuous cell line established from a spontaneous esophageal carcinoma of a rhesus monkey (Macaca mulata) by prolonged cocultivation with canine cells. A DNA transcript of the viral RNA hybridized to a high level and kinetic analysis indicated the presence of multiple copies of the viral genome in rhesus monkey DNA, showing that the virus is endogenous in this species. The rhesus monkey virus closely resembles, in several respects, an endogenous type C virus previously isolated from stumptailed macques (Macaca arctoides), aa species closely related to rhesus monkeys.