The behavioral response of infective stage juveniles of Neoaplectana carpocapsae to carbon dioxide was studied on an agar substrate. Exposure elicited a strongly positive response. Juveniles exhibited characteristic rearing and lateral motions of the head in migrating up the CO 2 gradient and aggregating at the source. The directed movement observed was attributed to klinotactic orientation. Maximum attraction was recorded at a flow rate of 4.1 ml/h; the range of attraction extended to 5 cm. Juvenile aggregations of approximately 60 and 90% were recorded within 30 and 50 mm, respectively, from a 2 cm distance. The pronounced response of N. carpocapsae to CO 2 suggests that this compound aids host finding.