An insecticide, diflubenzuron, suppressed 3rd- and 4th-instar larvae of the Colorado potato beetle, Leptinotarsa decemlineata (Say), with reduced effect on the tachinid parasite, Doryphorophaga doryphorae (Riley), developing within the larvae. Concentrations applied to potato foliage were 50, 100, 300, and 500 mg/L. In laboratory and greenhouse tests, parasite survival was not adversely affected when the 3rd instars were treated with 50 mg/L diflubenzuron but survival decreased to 0 and 4% at 300 and 500 mg/L. Survival and emergence of the parasite from treated 4th instars were not adversely affected at any concentration tested. Fourth-instar beetle larvae were much more tolerant of diflubenzuron than were the 3rd instars. Neither fertility nor the ability to parasitize beetle larvae was adversely affected in adult parasites emerging from treated 3rd or 4th instars. Adult beetles developing from treated larvae mated normally and laid fertile eggs. Foliage consumption by the beetle larvae decreased progressively as dosage of diflubenzuron increased but only slowly once the dosage increased beyond 100 mg/L.