Inversion of the 4.2-kb C segment flanked by 0.6-kb inverted repeats on the bacteriophage P1 genome is mediated by the P1-encoded site-specific cin recombinase. The cin gene lies adjacent to the C segment and the C inversion cross-over sites cixL and cixR are at the external ends of the inverted repeats. We have sequenced the DNA containing the cin gene and these cix sites. The cin structural gene consists of 561 nucleotides and terminates at the inverted repeat end where the cixL site is located. Only two nucleotides in the cixL region differ from those in the cixR and they are within the cin TAA stop codon. The cin promoter was localized by transposon mutagenesis within a 0.1-kb segment, which contains probable promoter sequences overlapping with a 'pseudo-cix' sequence cixPp. In a particular mutant, integration of an IS1-flanked transposon into the cin control region promoted weak expression of the cin gene. The cin and cix sequences show homology with corresponding, functionally related sequences for H inversion in Salmonella and with cross-over sites for G inversion in phage Mu. Based on a comparison of the DNA sequences and of the gene organizations, a possible evolutionary relationship between these three inversion systems and the possible significance of the cixPp sequence in the cin promoter are discussed.