Mutant C4, a poor plaque-forming mutant of Streptococcus mutans 6715-HSR, was employed to obtain isolates resembling the parent strain (a plaque former). Seventeen presumptive revertants, as identified by colonial morphology, were isolated from mutant C4 after enrichment cycles in a sucrose-glass beads medium. These isolates displayed properties which resembled the parent in ability to produce plaque, patterns of fermentation, and resistance to streptomycin. In a detailed study, five selected isolates were found to be similar to the parent type 6715-HSR with respect to content of the serotype antigen, sucrose- or dextran-induced cell aggregation, glucosyltransferase and adherence activities, and cariogenicity. Thus, in selection for revertants to parental colonial morphology, the pleiotropic changes in plaque formation, adherence, glycosyltransferase activity, and virulence demonstrated by C4 all concomitantly reverted to their parental phenotypes.