Polymerase chain reaction analysis of a large collection of bacteriophages with T-even morphology revealed four phages that are distantly related to all the others. The genomes of these pseudo T-even phages hybridized under stringent conditions to only a limited portion of the T4 genome that encodes virus head, head-to-tail joining and contractile tail genes. Except for this region, no extensive hybridization was detected between most pairs of the different pseudo T-even genomes. Sequencing of this conserved region of the pseudo T-even phage RB49 revealed substantial nucleotide sequence divergence from T4 (approximately 30% to 40%), and random genomic sequencing of this phage indicated that more than a third of its sequences had no detectable homology to T4. Among those sequences related to the T-even genes were virion structural components including the constituents of the phage base plate. Only a few sequences had homology to T4 early functions; these included ribonucleotide diphosphatase reductase, DNA ligase and the large subunit of DNA topoisomerase. The genomes of the pseudo T-even phage were digested by restriction enzymes that are unable to digest the T-even DNAs which contain glucosylated hydroxymethyl-cytosine residues. This suggests that only limited nucleotide modifications must be present in the pseudo T-even genomes. Conservation of much of the morphogenetic region of these diverse phage genomes may reflect particularly strong sequence constraints on these gene products. However, other explanations are considered, including the possibility that the various morphogenetic segments were acquired by the pseudo T-even genomes by modular evolution. These results support the notion that phage evolution may proceed within a network of both closely and distantly related genomes.