The sequences responsible for the proper partition of the P7 plasmid prophage to daughter cells lie within a discrete block of non-similarity between P7 and its close relative P1. The DNA sequence of the P7 region was determined. A segment with near identity to the replication (rep) region of P1 is followed by sequences (P7 par) that are clearly related to but very divergent from the P1 partition region. Subcloning was used to define the ends of the functional P7 partition region. It begins with a transcription promoter followed by two large open reading frames, parA and parB, that overlap by a single base and are read in the same direction. The genes direct the synthesis of two proteins, P7 ParA and ParB, with apparent Mr of 44,000 and 37,000. Specific frameshift mutations were introduced into the two genes. Each mutation blocked plasmid partition and both were complemented when the P7 ParA and ParB proteins were supplied in trans. The amino acid sequences of the P7 proteins show strong similarities to the P1 ParA and ParB proteins. However, the DNA sequences of the P7 and P1 open reading frames are remarkably divergent, largely caused by variability at the third positions in the codons. Interspecific complementation tests showed that the P7 proteins are unable to complement P1 parA or parB mutants, and the P1 proteins fail to complement the P7 mutations. Downstream from the P7 parB open reading frame is a sequence that conserves 27 of the 34 base-pairs of the P1 partition site parS. Unlike the P1 parS site, the P7 equivalent does not contain as extensive an inverted repeat. The heptamer sequences that define ParB binding sites within P1 parS are represented in P7 but differ from it by one base. A related sequence that coincides with the secondary ParB binding site within the P1 incB sequences is present nearby. Other sequences within the P7 incB region are rather different from their P1 counterparts. The basis for the major differences in specificity of the P1 and P7 par components is discussed. Comparison of the P1 and P7 sequences, and the nature of the junctions between similar and different sequences, suggest that the phages could have evolved by the pickup of divergent cassettes by recombination.