A mutant of Streptococcus mutans, GS-5, which differed in extracellular polysaccharide (EPS) produced from sucrose, was used to study the role of EPS in the production of dental caries. The mutant proved to be identical to the parent strain in sugar fermentation, growth rate, and serotype. Strain GS-5 synthesized an EPS, which in electron micrographs appeared to be of fibrillar structure, whereas the mutant produced no fibrillar material but only a globular EPS. Analysis of the EPS revealed that about 30% of the glucose units in the GS-5 polymer carried (1-3)-like bonds either as branch points or as part of the linear backbone and that the mutant material contained only about 3% of these linkages. When grown in sucrose broth, the proportion of the mutant culture adherent to the glass vessel was dramatically less than that of the parent strain. Caries scores produced in conventional rats by the mutant were significantly lower than those obtained with the parent strain. Since the only difference discovered between strain GS-5 and the mutant was the inability of the mutant to synthesize either a fibrillar EPS or an EPS with more than about 3% (1-3)-like linkages, it was concluded that the fibrillar EPS of strain GS-5 contained about 30% (1-3)-like linkages and was necessary for adherence of the bacteria to surfaces and for production of dental caries in test animals.