In the insect Manduca sexta the genitalia on the terminal abdominal segments are sexually dimorphic structures but they arise during metamorphosis from segments that are monomorphic in the larva. The motoneurons in the terminal ganglion that innervate these structures were examined by cobalt backfills of peripheral nerves. In the larval stage the population of motoneurons innervating the terminal segments was identical in both sexes. By contrast, the motoneuron populations in the terminal ganglia of adult males and females were strikingly different. No new motoneurons were produced during metamorphosis. Rather, this difference was the result of sex-specific cell death which occurred primarily during the early stages of adult differentiation. Possible mechanisms underlying this sex-specific degeneration of neurons are discussed.