54 pre-school children who had no caries on the proximal surfaces between their primary molars, were examined every 6 months during V-11/2 year. Plaque samples were collected from the areas mentioned above. Samples from the right side were used for cultures and samples from the left side for gram-stained smears. From the cultures, the following ratios were determined: (1) number of aerobic/anaerobic microorganisms, (2) number of Streptococcus mutans/number of microaerophilic streptococci, (3) number of microorganisms producing intracellular polysac-charide/total number of microorganisms, and (4) number of streptococcitotal number of microorganisms. In the gram-stained smears the distribution of different gram-positive and gram-negative microorganisms were studied. The only parameter which changed in connection with caries development was the ratio aerobic/anaerobic microorganisms. The ratio ranging from 7.5 to 139.5% prior to the period of caries development decreased to between 0.5 and 43.5% in connection with caries development. With regard to the other ratios between viable counts, no correlation could be established with the development of caries. Similarly, no changes in the composition of the microflora were observed in the gram-stained smears.