Second-instar gypsy moth,Lymantria dispar(L.), larvae parasitized byApanteles melanoscelus(Ratzeburg) are substantially more susceptible to carbaryl than unparasitized larvae. The 3-day LD95values for parasitized and unparasitized larvae were 0.143 and 0.469 μg carbaryl, respectively. During the next 12 days, the carbaryl-treated parasitized group showed higher mortality than treated unparasitized larvae, probably because of an interaction between the effects of parasitism and the sublethal effects of carbaryl. The time from the egg stage to cocoon formation by the parasite varied from 11.2 days in untreated gypsy moth larvae to 14.1 in insecticide-treated specimens, with the increase in time being proportional to the carbaryl dosage. The mortality due to parasite action alone occurred over a 24-day period, beginning on the 15th day after carbaryl treatment; there was no indication of insecticide-parasite lethal interaction during this period. Other effects of the treatments on gypsy moth were alteration of the sex ratio and an increased incidence of supernumerary molting.