The toxicity of monocrotophos, aldicarb, and disulfoton administered topically and orally to adult Campoletis perdistinctus (Viereck) and the toxicity to the parasite or cotton plants treated with the same compounds were studied. Monocrotophos was more toxic than adicarb or disulfoton when administered topically or orally. Likewise, when caged on entire greenhouse-grown plants treated on the stem, with Monocrotophos, adults were killed for at least 14 days after application; adults fed nectar collected from similar plants were killed for 6 weeks. In similar studies parasite adults caged on cotton plants treated on the stem with aldicarb were killed at 3 and 14 days after treatment when caged on entire plants. However, adults fed nectar from these plants were killed only at 3 days after treatment. Adults of C. perdistintus caged on entire plants in the greenhouse after soil treatment with aldicarb were killed 3 days after treatment, but adults caged on leaf surfaces and nectar from these plants were killed up to 7 days after treatment. Disul foton treated plants did not appear to be toxic in any or the studies. Adult parasites were not killed by fumigant action of any of the 3 systemic insecticides. Stem applications of monocrotophos and soil applications of aldicarb to cotton in the field did not appear to harm adult C. perditinctus, though nectar collected from treated plants killed some parasites.