The distribution, dispersal, and population growth of an ectoparasitic nematode,Noctuidonema guyanenseRemilIet&Silvain, were studied on mating pairs ofSpodoptera frugiperda(J. E. Smith) in the laboratory. Nematode populations were the greatest on terminal abdominal segments of wildS.frugiperdamales. Eggs and neonates were predominant on abdominal segments eight to ten, whereas juveniles were distributed widely along a host's abdomen and thorax. Although female nematodes moved from infested male moths to noninfested female moths within 15 min after initiation of host mating, male and immature nematodes often did not transfer until 1.5–2.0 h after host copulation began. Nematodes migrated anteriorly to the fifth abdominal segment of new hosts in 4–7 h. Forty-eight percent of the host-matings resulted in the transfer of an average of 5.3 nematodes to new hosts. Populations ofN.guyanenseon newly infested female moths increased to an average of 400 nematodes within 176 h after the initiation of mating by hosts. About 80% of the nematodes that transferred to new hosts were juveniles.