1. Bacteriophage active against four species of bacteria was found in a salt solution extract of house flies.
2. A growth-inhibiting principle, not bacteriophage, active against four other species of bacteria was found to be present in the same extract.
3. An attempt to secure streptococcus bacteriophage by feeding to flies a streptococcus susceptible to the inhibitor but not to the bacteriophage of this filtrate was unsuccessful, indicating that the two activities are quite unrelated.