Results from comparisons of deoxyribonucleic acid (DNA) from several classes of bacteriophages suggest that most phage chromosomes contain either a homogeneous distribution of nucleotides or are made up of a few, rather large segments of different quanine plus cytosine (G + C) contents which are internally homogeneous. Among those temperate phages tested, most contained segmented DNA. Comparisons of sequence similarities among segments from lambdoid phage DNA species revealed the following order in relatedness to lambda: 82 (and 434) > 21 > 424 > phi80. Most common sequences are found in the highest G + C segments, which in lambda contain head and tail genes. Hybridization tests with lambda and 186 or P2 DNA species verified that the lambdoids and 186 and P2 belong to two distinct groups. There are fewer homologous sequences between the DNA species of coliphages lambda and P2 or 186 than there are between the DNA species of coliphage lambda and salmonella phage P22.