The dnaB gene product of Escherischia coli is required for multiplication of temperate phage P2. At 37 C in dnaB-ts mutnats, P2 will not plaque and gives a very small burst of progeny. P2 mutants have been isolated which can grow well enough to plaque under these conditions. This type of phage mutant is cis dominant, and one such mutant (P2rlb1) has been mapped near the left end of the early gene B and to the right of the cox4 (excision) mutation. The rlb1 mutation does not lie at the replication origin, but may affect transcription in the early region, which includes the replication origin. It may also represent a site on the P2 DNA which interacts with the dnaB gene product.