The genome of the tailed temperate coliphage P2 (Myoviridae) contains some genes that probably are horizontally transferred additions to the genome. One of these genes, the Z/fun gene, was recently found intact in the genome of Neisseria meningitidis. We have investigated the presence of P2-like phages, and the genetic variation at the position corresponding to the phage P2 Z/fun locus, in the Escherichia coli reference collection (ECOR). P2-like phages are common in E. coli since they are present in about 30% of the ECOR strains. Hybridizations and PCR amplifications indicate that the overall variation among these phages is small. Amplification of the region corresponding to the phage P2 Z/fun locus in 11 prophages revealed that this is a multivariable locus. Sequencing of the region resulted in 10 completely different sequences but with a similar high AT-content as the Z/fun gene. All sequences contained at least one open reading frame with good transcription and translation signals. All sequences were also surrounded by a highly similar, previously undiscovered, inverted repeat (IR). We also found this IR in genetically unstable regions in pathogenic enterobacteria. This demonstrates that P2-like phages are important factors in the evolution of bacteria, not only because they carry a diversity of lysogenic conversion genes but also because they can act as vectors for single genes. The genes found between the IRs have unknown functions, and only a few clearly similar genes have been found in other bacteria.