The general morphology of the external genitalia was examined in 43 common marmosets (Callithrix jacchus) ranging in age from newborn to adult. At birth, the scrotum was a small irregular fold of skin on either side of the caudal aspect of the penis. The testes were not present in the scrotum until 8--11 months of age. The scrotum covered the penis in the adult male, was devoid of fur, was covered by pearly while nodules, and presented a median raphe. In the young female, the pudendal pad closely resembled the scrotum of the male. The vulva was pendulous, and the small vestibular opening was located near its most ventral aspect and closely resembled the preputial opening of the male. In the adult female, the pudendal pad was pendulous, was studded with white nodules, and closely resembled the scrotum of the male. Sex determination by casual observation resulted in numerous errors. Accurate sex determination was based on differences in the preputial and vestibular openings, demonstration of the glans penis in the prepuce, and palpation of the testes in the scrotum or inguinal region.