When Escherichia coli cells that had been irradiated with ultraviolet light were infected with bacteriophage phi 80, five major (pE, pB, pA, pC and pD) and two minor (pU and pV) proteins were found to be synthesized during early stages of infection. The genes coding for the five major proteins were mapped on the phi 80 chromosome using various deletion mutants which lacked the capacity to synthesize some or all the major proteins. The size and positions of all the deletions were determined by gel electrophoresis of EcoRi digests of phage DNA and by electron microscopy of heteroduplexes between DNAs of the deletion and wild-type phage. The five major proteins designated pE(25K), pB(40K), pA(45K), pC(34K) and pD(31K) were shown to be encoded in this order presumably by a single operon that was located at 60.2--67.4% on the phi 80 genome. These proteins were found to be involved in phage recombination. The absence of pE or pB resulted in a Red- phenotype and the absence of three proteins (pE, pB and pA) resulted in a Fec- phenotype. The exact positions of the genes for the minor proteins pU(29K) and pV(26K) have not been determined.