The influence of hydroprene (Altozar®), a juvenile hormone mimic (JHM), on parasitized and unparasitized cabbage looper,Trichoplusia ni(Hübner), was studied. Larvae were reared on standard wheat germ diet until early 4th stadium then on diet containing 0.1, 1, 2 and 4 ppm of hydroprene. Larval development from the 4th to the 5th stadium was essentially the same for all treatments. A decided influence was found in the development of the unparasitized 5th stage larvae at the 1, 2 and 4 ppm treatments of the JHM. Longer time intervals were required to spin, for pupation and adult eclosion. No adults were obtained from 2 and 4 ppm hydroprene treatments and only 2.5% survived from the 1 ppm treatment.T. nilarvae parasitized byHyposoter exiguae(Viereck) developed the same on JHM and control diets. Parasites emerged from hosts on both diets at the same time and immediately began to spin cocoons. Adult eclosion and survival was no different as a result of host diet. When JHM was topically applied at 2 and 20 micrograms prior to insect parasite emergence, development was extended and adult mortality ranged from 5–95%.JHMs generally disrupt insect development prior to the larval pupal molt. Since the parasite's last larval molt occurred after the host ceased feeding, apparently the hydroprene was not available in sufficient quantity during the parasite's last instar to influence its development. Topical application of the JHM near the time of parasite emergence allowed sufficient material to reach the parasite during its last instar and influenced development. These data indicate the importance of knowing developmental patterns of beneficial insects to determine the potential impact of a JHM applied for insect control.