Strains of houseflies were obtained from a refuse tip and 34 farms selected at random from Eastern England in 1984. Resistance was assessed to four insecticides that had been in use for fly control in the UK. Dose-response data were obtained by topical application of the insecticides and, in addition, a resistance test method was used in which flies were allowed to feed on sugar coated with either methomyl or azamethiphos. For both methods, the knock-down (KD) after 48 h was used as a measure of response. The ranges of resistance factors for the topical application method were, for methomyl, 1·1–15·9 at KD50 and 2·1–17·0 at KD95; for azamethiphos, 2·2–36·9 at KD50 and 4·5–88·2 at KD95; for pyrethrins + piperonyl butoxide, 0·9–6·8 at KD50 and 1·4–13·4 at KD95; and for permethrin, 1·7–34·8 at KD50 and 2·2–112·5 at KD95. The ranges of the resistance factors for the feeding tests were, for methomyl, 2·1–39·8 at KD50 and 2·4–64·0 at KD95, and, for azamethiphos, 5·6–51·7 at KD50 and 8·8–92·0 at KD95.