Escherichia coli was isolated from feral house mice (Mus domesticus) during the course of a mouse plague in the state of Victoria, Australia. The isolates were characterized for the production of colicins and their resistance to the co-occurring colicins. Of the 447 isolates examined, 59% were found to be colicinogenic. Phenotypic and PCR-based genotypic methods were used to determine the types of colicins being produced. Colicin E2 was the most common, representing 27% of the colicin-producing isolates. Colicin Ia was produced by 3% of the colicinogenic isolates. The remaining colicins could not be identified, but phenotypic and PCR data argue that at least nine different colicin types are present in this collection of E. coli. The frequency of colicinogenic isolates declined from 71% to 43% over the 7 months of the study VSports手机版. All colicin types appeared to decline in frequency. Concurrently, the resistance of isolates to colicin E2 increased from about 50% to 70%. Two hypotheses are proposed to explain the decline in the frequency of colicinogeny in this population of E. coli. The first relates to the within-host interactions occurring among colicinogenic, colicin-susceptible and colicin-resistant populations within a host. The second relates to the among-host interactions between susceptible and colicinogenic populations and the effect of host population densities on these interactions. .