Evidence is presented that bacteriophage P7 specifies an analog of the E. coli DNA replication protein, dnaB. As in the related bacteriophage P1 (D'Ari et al., 1975; Ogawa, 1975), in lysogens of P7, the production of the analog protein is repressed and constitutive mutants could be isolated. Such constitutive mutants could suppress efficiently the thermosensitivity of several dnaB(ts) mutations and also rescue a strain carrying a dnaB amber mutation. While neither P7 nor the mutant P1bacban (defective in the structural gene ban) could suppress dnaB(ts) mutations efficiently, recombinants between these two phages could do so, indicating the presence of a functional dnaB analog gene (called sdb) on P7. In a dnaB amber strain suppressed by the presence of the constitutive mutant P7csb, bacteriophage lambda failed to replicate which is a further similarity between P7 and P1. P7csb mutants or P7-P1bacban recombinants were found to be less thermoresistant than P1bac1 suggesting that the P7-specified dnaB analog protein or its production is relatively less tolerant of temperatures above 37 degrees C.