Following infection by equivalent numbers of Streptococcus mutans serotypes a–e, gnotobiotic rats were monitored to determine which serotypes became implanted and their relative proportions when the animals were fed diets which contained glucose, sucrose or starch as the sole carbohydrate. S. mutans organisms were observed to colonize all sites sampled; the serotype prevalence differing considerably as the experiments progressed and according to the carbohydrate ingested by the animals. Serotype b organisms (rat strain FA-1) predominated in specimens from all sites. Serotype d organisms (strain 6715), frequently employed in caries studies involving the use of sucrose diets, required sucrose to colonize and remain on teeth in the gnotobiotic animals studied. Furthermore, colonization of the animals by S. mutans was observed to stimulate detectable levels of 7-S antibody to S. mutans in seru