none The study was conducted to determine how various factors, including learning, influence the host-selection behavior of the larval parasitoid Cotesia (= Apanteles) marginiventris (Cresson). Frass, silk, and feeding danage from Spodoptera frugiperda (J.E. Smith) larvae elicited visits and antennal examinations by females of the parasitoid. Learning played a role in the effective response to these stimuli in that previous contact experience with hosts and/or chewed leaves, exuviae, frass, or hemolymph significantly enhanced the response of the parasitoid. Experienced parasitoids were more active and spent less time at rest than inexperienced parasitoids. Previous experience was also found to be of significant importance in the ability of the parasitoid to discriminate between parasitized and unparasitized host larvae.