The response of maleTrichoplusia ni(Hübner) released in the field was affected by the prerelease conditions of acclimation. Thus, 37 percent of the moths held at near-ambient out-of-door conditions were captured in pheromone-baited traps; only 13 percent of the moths held at a constant 11°C were recovered. Prerelease acclimation could therefore significantly affect success or failure of a program of population suppression involving use of sterile male cabbage loopers in a relatively small area.