The occurrence of mutans streptococci species was investigated in Tanzanian infants aged between 1.0 and 3.5 years with rampant caries and without caries using biochemical and immunological techniques. These infants came from Turiani and Singida, where the caries prevalence for this age group was 12.8% and 1.6% respectively. Streptococcus mutans was found to be the only mutans streptococcal species. The other species, including the previously reported Streptococcus rattus, were not found. Consequently, the differences in caries experience observed between and within the two populations could not be explained by differences in mutans streptococcal species.