A nitrosoguanidine-induced mutant, designated UAB90, of Streptococcus mutans PS14 (serotype c) strain UAB62, was identified on the basis of its unique colony morphology and isolated on brain heart infusion agar. Other mutants displaying similar colony morphologies on brain heart infusion agar were isolated after ethyl methane sulfonate mutagenesis of UAB62 and S. mutans 6715 (serotype g) strains UAB61 and UAB66, and these were found to exhibit abnormalities in cell morphology, chain length, or both. All mutants were examined further for (i) adherence and aggregation after overnight growth in medium containing sucrose, (ii) growth and aggregation in brain heart infusion broth and medium containing glucose, (iii) aggregation of nongrowing cells in the presence of 2 mg of sucrose per ml or 200 mug of dextran per ml, (iv) dextranase activity, and (v) ease of cell lysis. Mutants isolated included several with long chains of enlarged cocci, and two of these strains, UAB261 and UAB433, along with UAB90, were more susceptible to cell lysis than were their parents. UAB261, isolated from UAB62, maintained other parental characteristics, whereas UAB433, isolated from UAB66, lost its ability to aggregate in the presence of either sucrose or dextran. The "fragile" mutant UAB90 was particularly useful in the isolation of high-molecular-weight DNA for early gene cloning experiments by our laboratory. Two other cell morphology mutants, UAB272 from UAB66 and UAB289 from UAB61, did not lyse better than their parents, but both lacked measurable dextranase activity. A final mutant, UAB276 from UAB66, displayed only increased chain length without apparent cell morphology variations. Chains produced by this mutant were up to 10 times longer than those produced by UAB66. UAB276 lysed slightly less well than its parent but retained all other wild-type characteristics examined.