none Toxic and behavior-modifying actions of several formulated insecticides were determined for Microplitis croceipes (Cresson), a braconid parasitoid of Heliothis spp. Exposure of adult parasitoids to cotton plants sprayed at recommended field rates with a pyrethroid/formamidine mixture (fenvalerate/chlordimeform) resulted in significantly higher mortality rates (10.4 to 22.6 percent) than in controls. Exposure to the carbamate thiodicarb resulted in similar rates of mortality. Only methomyl, a type of carbamate different from thiodicarb, caused mortality significantly higher than all other treatments, ranging to about 70%. The results suggest that the actions of insecticides, other than those of direct toxicity, may be important on beneficial parasitoids. Behaviors, such as flight activity and foraging, may be altered by even relatively non-toxic insecticides, thus potentially modifying the effectiveness of natural enemies.