The role of living gastrointestinal bacteria in the growth and development of face fly (Musca autumnalisDeGeer) larvae in bovine feces was evaluated. Egg-to-pupa survival and mean pupal weight were lower for larvae grown in steam-sterilized feces than for larvae grown in unsterilized feces (4.6 versus 28.0% and 15.3 versus 26.1 mg, respectively). However, larval development was not retarded in sterilized feces that had been inoculated with bacteria collected from the bovine rumen or with a mixture of 11 bacterial isolates representing the major bacterial groups from the gastrointestinal tract. When tested individually, all of the isolates tended to improve larvae survival in sterilized feces, but no single isolate provided for survival rates equivalent to those observed with the mixed inoculum. Survival rates were greatest when sterilized feces were inoculated with strains ofEscherichia coliorLactobacillus plantarumwhich gave survival rates of 57.2 and 32.8% of the unsterilized control, respectively. These data suggest that the presence of living bacteria in bovine feces is essential to normal growth and development of face fly larvae.