Observations on the sexual behavior of 2 species of spider monkeys, Ateles belzebuth and A. geoffroyi indicate that primary sexual behavior is diurnal and, in contrast to groups of Saimiri sciureus sympatric with A. belzebuth at a South American study site, non-seasonal. The copulatory posture taken by the male spider monkey during penile insertion is different from that described for any other primate. Behavioral data strongly support the hypothesis that the transmission of information concerning female sexual receptivity is via chemical stimuli, and that the primary function of the female spider monkey’s hypertrophied clitoris is the deposition of urinary scent