Abstract Adult house flies (Musca domestica) were susceptible (94-100% mortality) to Beauveria bassiana when conidia of the Hf88 isolate were applied to plywood boards at 107 conidia/cm2; a starch dust formulation was more effective than a liquid suspension. Adult flies were also susceptible (90-99% mortality) to this isolate when they were con fined with treated water (108 conidia/ml) or food (108 conidia/100 mg). House fly larvae were not affected by treatments with up to 108 conidia/cm3 of rearing medium. A 2-year survey of house flies cm from New York dairies indicated that most natural infections of house flies occurred in September and October, although prevalence rates never exceeded 1%. Thirteen single-fly isolates obtained during this survey were compared with the Hf88 isolate for virulence against flies; the 2 most virulent isolates were slightly more virulent for flies than for the fly parasitoid Muscidifurax raptor. Incorporation of conidia into a sucrose bait (108 conidia/100 mg) in cages gave high levels of house fly control (78-100% mortality) 5 days after exposure.