Genes of the right arm of the pi80 phage chromosome were characterized by studying lytic and antigenic activities and replication ability of mutant phages and by electron microscopic examination of their lysates. It is shown that the genes 14, 15 and 16 are early, and the genes 17, 18 and 19 are late. The functions of four genes are determined. The gene 13 is responsible for antigenic activity and for phage host-cell specificity; the gene 17 is a regulator of the late functions; the gene 18 product affects the cell membrane; the gene 19 controls the synthesis of phage endolysine. It is suggested to redesignate the pi80 phage genes 13, 17, 18 and 19 by the letters J, Q, S and R respectively, because these genes fulfil the same functions as the lambda genes J, Q, S and R.