Female Japanese macaques (Macaca fuscata), in certain captive and free-ranging populations, frequently engage in same-sex mounting. Traditionally, same-sex mounting interactions in animals have been characterized as "sociosexual," that is, sexual in terms of their superficial form, but enacted to facilitate adaptive social goals. Sexual motivation is rarely ascribed to sociosexual interactions because their adaptive functions are often seen as their primary purpose, thus diminishing, or even negating, any sexual component that such activity might have. A substantial number of studies indicate that female-female mounting in Japanese macaques is not a sociosexual behavior. In contrast, several lines of circumstantial evidence suggest that these interactions are, indeed, sexual. In this study, we documented patterns of vulvar, perineal, and anal (VPA) stimulation during same-sex mounting in female Japanese macaques. During the majority of female-female mounts analyzed, female mounters engaged in repetitive VPA stimulation. Two forms of VPA manipulation by female mounters were observed. First, while sitting in a jockey-style position on the mountee, a female mounter would rub her VPA region against the mountee's back. Males never executed this type of mount posture or pelvic movement. Second, female mounters rubbed their VPA regions with their tails during same-sex mounts. Females mounters moved their tails in a voluntary, sex-specific manner and were never observed to do so in non-sexual contexts. Given the primary role of the VPA regions in mediating sexual response in primates, the results of this research provide direct evidence bearing on the sexual nature of female-female mounting in Japanese macaques and give further support for the conclusion that the term "homosexual behavior" is an appropriate label for these interactions.