The persistence of Escherichia coli in artificially fed larvae was examined for up to 48 h after ingestion by house flies, Musca domestica L., and stable flies, Stomoxys calcitrans (L.). The rate of change in the E. coli load was similar for both species for up to 5 h after ingestion. Up to 48 h after ingestion, abundance of E. coli declined in immature house flies but remained constant in immature stable flies. When different E. coli concentrations were fed to larvae, the abundance of E. coli increased in stable fly larvae regardless of the initial concentration. The E. coli load in house fly larvae increased when larvae were fed a low concentration of bacteria, but it declined when larvae were fed a high concentration of bacteria. Survival of house fly and stable fly larvae averaged 62 and 25%, respectively, when reared on pure E. coli cultures. These observations suggest that house fly larvae digest E. coli and use it as a food source but stable fly larvae do not.