Three phages ofStreptococcus mutans were obtained and partially characterized. The three phages, designated M102, e10, and f1, were found to be strictly lytic, with host ranges restricted to only serotype c, e, and f strains of this species, respectively. Phage sensitivity was not correlated with the presence of plasmids, at least in host strains of serotypes c and e. Each phage produced clear plaques in a number of standard media, even in the presence of sucrose, indicating that the extracellular glucan polysaccharides (mutan) produced by the hosts from this substrate do not prevent phage adsorption and growth. The phages were similar in size and morphology, having icosahedral heads and long (283-287 nm), flexible, noncontractile tails. The genome of each phage was found to consist of linear, double-stranded DNA, 31-35 kb in length, with a base composition of 37-38% G+C. Restricting phage DNAs with four enzymes produced fragment patterns unique to each phage, but common bands between M102 and e10 and between e10 and f1 were produced byBamHI. Labeled e10 and M102 DNAs hybridized strongly with all three phage DNAs, indicating that they share some common sequences. The three phages appear to be more similar than expected and probably evolved from a common ancestor.