Bacteriophage P1 lysogenizes Escherichia coli as a unit-copy plasmid. We have undertaken to define the plasmid-encoded elements implicated in P1 plasmid maintenance. We show that a 2081 base-pair fragment of the 90,000 base P1 plasmid confers the capacity for controlled plasmid replication. DNA sequence analysis reveals several open reading frames in this fragment. The largest is shown to encode a 32,000 Mr protein required for plasmid replication. The corresponding gene, repA, has been identified genetically. A set of five 19 base-pair repeats is located upstream from repA; a set of nine similar repeats is located immediately downstream from repA. Each set of repeats, when cloned into pBR322, exerts incompatibility towards a P1 replicon. The upstream set, designated incC, consists of direct repeats that are spaced about two turns of the DNA helix apart; the downstream set, designated incA, consists of nine repeats arranged three in one orientation and six in the other. Spacing between incA repeats were three or four turns of the helix apart. The organization of the plasmid maintenance regions of P1 and the unit-copy sex factor plasmid, F, is strikingly similar. Although the DNA sequences of this region in the two plasmids exhibit little homology, a 9 base-pair sequence that appears four times in the origin region of members of the Enterobacteriaceae also occurs twice as direct repeats in similar positions in P1 and F. This sequence, where it occurs in E. coli, has been postulated to be the binding site for the essential replication protein determined by dnaA. The dnaA protein appears not to be essential for the replication of either plasmid; therefore, the function of the sequence in P1 and F may be regulatory.