The relative susceptibility to dieldrin, dimethoate, trichlorfon, dichlorvos, and naled of mature 3rd-instar larvae and 3- to 4-day-old females of the house fly, Musca domestica L., Fannia pusio (Wiedemann), Boettcherisca peregrina (Robineau-Desvoidy), Chrysomya megacephala (F.), Chrysomya mfifacies Macquart, and Phaenicia cuprina (Wiedemann) was investigated. The latent toxicity of these insecticides to each species was also determined. Median-lethal dosages and dosages causing 90% mortality with respective fiducial limits were calculated by probit analysis. Larvae of B. peregrina, C. megacephala, C. rufifacies, and P. cuprina were more resistant to the insecticides than were the corresponding adults. However, the larva of M. domestica was more susceptible than the adult to trichlorfon. Of the 5 toxicants tested, dieldrin was the only one to exhibit latent toxicity. Larvae of C. megacephala, C. rufifacies, and P. cuprina were very variable in their response to this insecticide. Larvae of M. domestica and B. peregrina were unaffected by dieldrin at dosages of 88,100 and 55,500 µg/g, respectively.