The genomes of pilus-specific, lipid-containing phages PR4 and PR772 were studied electron microscopically. An identical mol. wt. of 10.9 x 10(6) was obtained. The genomes are unique (non-permuted) and have cohesive ends. From the similarities in size and denaturation maps of the genomes and failue to demonstrate non-homology in heteroduplexes, reported morphological ambiguities were clarified. The known serological difference between the phages could not be related to non-homology of their genomes. It is concluded that phages PR4 and PR772 are the same phage.