Abstract The oral implantation of salivary agglutination-positive and -negative mutans streptococci was studied using streptomycin resistant (Str R) organisms. Str R Streptococcus mutans strains Ingbritt and NCTC 10449 are agglutinated by rat saliva and the Str R strains Streptococcus sobrinus 6715-13 and Strep. mutans GS5 are not. Four groups of Sprague-Dawley rats were inoculated orally with each organism (one per group) and fed a sucrose diet. A further two groups of animals were similarly inoculated with either the agglutination-positive Strep. mutans Ingbritt or the agglutination-negative Strep. sobrinus 6715-13 and fed a glucose diet. Str R streptococci were recovered from smooth-surface dental plaque of all animals on the sucrose diet with no significant difference in the recovery of agglutination-positive Strep. mutans strains Ingbritt and NCTC 10449 and agglutination-negative Strep. mutans GS5. However, the recovery of agglutination-negative Strep. sobrinus 6715-13 from smooth-surface plaque of animals on either the sucrose or the glucose diets was significantly lower than that of the other strains. Agglutination-positive Strep. mutans Ingbritt colonized smooth enamel surfaces of animals on the sucrose and the glucose diets in numbers that were not significantly different. However, the colonization of such surfaces by agglutination-negative Strep. sobrinus 6715-13 was significantly enhanced by the sucrose diet. Agglutination-positive and -negative Str R mutans streptococci were recovered from fissure plaque of all inoculated sucrose-fed animals in numbers that were not significantly different. Successful colonization of smooth enamel surfaces by the Str R streptococci resulted in increased smooth-surface caries. However, successful colonization of fissures by the Str R strains did not increase the incidence of fissure caries over that obtained with indigenous organisms. The results suggest that agglutination by saliva is not a characteristic that limits mutans streptococci from colonizing molar smooth surfaces or fissures in the rat.