An evaluation of the infectivity of the newly emerged adult housefly is based on a quantitative bacteriological study of the following:
1.Feces and meconium of flies reared with a single species of human pathogen or with a multiple saprophytic flora;
2.Cumulative sampling of feces from flies maintained aseptically for 2 days;
3.Testing of digestive tract;
4.Sampling of bacteria on the surface and within the fly.
Data for the first three categories indicate that the fly's gut contains very few bacteria. This is further evidence for a general process of gut sterilization which commences before pupation and is completed by the time of eclosion. The data for the fourth category show that surface contaminants account for only a part of the remaining flora, the rest of which is retained internally at counts of about 100. The epidemiological implications of these findings are discussed.