Summary 1. Salmonella typhi and Shigella flexneri, introduced into CSMA, a normally contaminated larval medium, were recovered infrequently from larvae and not at all from adult houseflies. 2. Under the same conditions, Salmonella paratyphi B was recovered more frequently from larvae and adults, though from a smaller percentage of adults than larvae. 3. S. typhi, when introduced into a sterilised larval medium, probably underwent some multiplication, whether or not maggots were present. 4. Each of the above species, including Salmonella enteritidis, when introduced into a sterilised larval medium, was routinely recovered from larvae and pupae of various ages. Mature maggots typically harbored about 107 organisms. The counts of pupae from the second day until emergence were stabilized at 103 to 104. A small number of pupae were sterile or retained relatively few bacteria. 5. An explanation based on interspecific competition in the larval medium is suggested for the differential survival of these pathogens in fly stages breeding under natural conditions.