Three group I introns of bacteriophage T4 have been compared with respect to their sequence and structural properties. The introns include the td intervening sequence, as well as the two newly described introns in the nrdB and sunY genes of T4. The T4 introns are very closely related, containing phylogenetically conserved sequence elements that allow them to be folded into a core structure that is characteristic of eukaryotic group IA introns. Similarities extend outward to the exon sequences surrounding the three introns. All three introns contain open reading frames (ORFs). Although the intron ORFs are not homologous and occur at different positions, all three ORFs are looped-out of the structure models, with only the 3' ends of each of the ORFs extending into the secondary structure. This arrangement invites interesting speculations on the regulation of splicing by translation. The high degree of similarity between the T4 introns and the eukaryotic group I introns must reflect a common ancestry, resulting either from vertical acquisition of a primordial RNA element or from horizontal transfer.