Various insecticides for the control of the house fly Musca domestica L. were tested for compatibility with a biological control agent, the pupal parasitoid Spalangia endius Walker. Bioassays used the mode in which each organism was expected to be harmed by the insecticides, a surface contact bioassay for S. endius and a feeding bioassay for M. domestica. A Pesticide Compatibility Index (PCI) was created that allows comparison of LC₅₀ values when the mode of exposure to a pesticide differs. First LC₅₀ values were converted into units of prescribed dosages (LPR = LC₅₀-to-prescribed dosage ratio). This study used dosages from labels of granular baits. PCI is the ratio of LPR_{biological control agent} to LPR_pest. For these PCI values, order of compatibility with S. endius was spinosad > thiamethoxam > dinotefuran > methomyl > imidacloprid. That spinosad was better than imidacloprid or methomyl, both for parasitoid survival and for killing flies, is consistent with conclusions from the LC₅₀ values. Permethrin and nitenpyram were also tested, but their PCIs were not calculated. Permethrin is prescribed as a contact insecticide against flies rather than being consumed as a bait, and nitenpyram has not been formulated as a fly insecticide. Compared with the other insecticides in terms of LC₅₀ values, permethrin was moderately toxic to S. endius but one of the most toxic for M. domestica, whereas nitenpyram was least toxic for both S. endius and the flies.