The sexual behavior of the Arashiyama West troop, a natural semi-free-ranging troop of Japanese macaques, was studied during the 1973-1974 and 1974-1975 breeding seasons. The troop was transported intact from Japan to its current location in South Texas. There they have been free to move about in a 42.2-hectare electric fence enclosure. This report describes the relationships among age, aging, and sexual behavior. Males beginning at the age of 2.5 years were observed to series-mount estrous females in the double foot clasp mount position. Mounting in series in the double foot position is the normal pattern for Japanese macaque males. The ejaculation of semen with concomitant body movements indicative of orgasm begins at age 4.5 years at the earliest and continues until death. Males 4.5 and 5.5 years of age, sexually mature in the physiological sense, were not consistently sexually mature in the behavioral sense. The oldest male displayed traits that appear analogous to traits observed in aging human males. Females begin to experience estrus at 3.5 years of age. However, like pubescent males, pubescent females display behavioral patterns of sexual immaturity. The oldest female of the troop has remained sexually active. The attainment of sexual maturity by adulthood can be viewed as a learned process leading to efficiency and prowess, followed in old age by sexual involution.