A collection of 266 faecal isolates of Escherichia coli from humans was assayed for the production of mitomycin C-inducible bacteriocins and screened using a PCR-based method for the presence of eleven colicins and seven microcins. Eight different colicins were detected and all seven microcins. Of the strains examined, 38 % produced a bacteriocin, 24 % produced a colicin and 20 % produced a microcin. Of the 102 bacteriocin-producing strains, 42 % produced one type of bacteriocin, 41 % produced two, 16 % produced three and one strain was found to produce four different bacteriocins. Strains producing more than one bacteriocin were more likely to be members of E. coli genetic group B2 and less likely to belong to genetic groups A or D. Several of the bacteriocins were found to co-occur in a strain more often than would be expected by chance: microcins H47 and M; colicin la and microcin V; colicins B and M; colicins E1 and M; colicins E1 and Ia. No bacteriocins released as a consequence of cell lysis were found to co-associate more often than expected by chance. Three non-mutually exclusive hypotheses are presented that might explain the high frequency of multiple bacteriocin production in E. coli strains: (1) expanded killing range, (2) expanded receptor repertoire and (3) fitness benefits in different environments.