Tailed bacteriophages are the most abundant biological entities in marine environments. However, most of these marine phages are uncharacterized because few of their hosts have been cultivated. To learn more about such phages, we designed a set of degenerate PCR primers for phage T4 g23, which encodes the major capsid protein in all of the T4-type phages, an important family of the tailed phage. These primers were used to amplify g23-related sequences from diverse marine environments (fjords and bays of British Columbia, the eastern Gulf of Mexico, and the western Arctic Ocean) revealing a remarkable level of molecular diversity, which in some cases was correlated with morphological variation of the virions. Phylogenetic analysis showed that although some of these sequences were closely related to well studied subgroups of the T4-type phage, such as the T-evens, the majority of them belong to five previously uncharacterized subgroups. These data indicate that the host range of T4-type phages is much broader than previously imagined and that the laboratory isolate T4 belongs to a phage family that is extraordinarily widespread and diverse in the biosphere.Author contributions: J.F., F.T., and H.M.K. designed research; J.F. and F.T. performed research; C.A.S. contributed new reagents/analytic tools; J.F., C.A.S., and H.M.K. analyzed data; and J.F., C.A.S., and H.M.K. wrote the paper.This paper was submitted directly (Track II) to the PNAS office.Data deposition: The nucleotide sequences of g23 reported in this paper have been deposited in the GenBank database (accession nos. DQ105858-DQ105942).