Group I self-splicing introns are present in the td, nrdB and sunY genes of bacteriophage T4. We previously reported that whereas the td intron is present in T2, T4 and T6, the nrdB intron is present in T4 only. These studies, which argue in favor of introns as mobile genetic elements, have been extended by defining the distribution of all three T4 introns in a more comprehensive collection of T2, T4 and T6 isolates. The three major findings are as follows: First, all three introns are inconsistently distributed throughout the T-even phage family. Second, different T2 isolates have different intron complements, with T2H and T2L having no detectable introns. Third, the intron open reading frames are inherited or lost as a unit with their respective flanking intron core elements. Furthermore, exon sequences flanking sites where introns are inserted in the T4 td, sunY and nrdB genes were determined for all the different T-even isolates studied. Six of eighteen residues surrounding the junction sequences are identical. In contrast, a comprehensive comparison of exon sequences in intron plus and intron minus variants of the sunY gene indicate that sequence changes are concentrated around the site of intron occurrence. This apparent paradox may be resolved by hypothesizing that the recombination events responsible for intron acquisition or loss require a consensus sequence, while these same events result in sequence heterogeneity around the site.