Selective culture media were used for the isolation of microorganisms from the lingual developmental groove and the buccal surface of the upper right second deciduous molar tooth in 22 monkeys. Samples of pooled plaque were also cultured for streptococci. Some of the lingual grooves became carious during the study. Fusobacteria formed a greater percentage of the cultivable flora on the buccal surface but streptococci, and in particular strains of Streptococcus mutans serotype c, were more numerous in the lingual groove.