1. Acetylcholinesterase from the heads of a strain of houseflies selected for resistance to the carbamate insecticide methomyl, and from a methomyl-resistant field strain was found to be less sensitive to inhibition by methomyl than that from a susceptible strain. 2. The enzyme from resistant insects was also more tolerant to malaoxon, dichlorvos and bomyl but not to azamethiphos. 3. The decrease in sensitivity to inhibition appeared to be due to an increase in affinity for substrate.