Three sequential samples from the distal surface of the upper first premolars of ten 13- to 14-year-old children were collected over a period of 12 weeks. Viable bacterial counts were made on each sample and representative strains of the genera Streptococcus, Actinomyces, Veillonella, Neisseria, Fusobacterium, Rothia, Lactobacillus, Bacterionema, Eubacteria and Arachnia were identified on the basis of morphology, physiological tests, cell wall and acid end product analysis and serology. The qualitative composition of the 58 samples obtained from the 20 sampled areas examined was similar, although some surfaces were consistently free of specific organisms during the test period, this was particularly evident with the lactobacilli. The mean percentages of the predominant genera were Actinomyces 34.92, Streptococcus 22.9, Veillonella 13.07, and ‘bacteroides’ 7.97. Streptococcus mutans mean percentage was 2.17, Strep. sanguis 5.9, Strep. salivarius 0.67, Strep. milleri 0.51, A. israelii 16.5 and A. viscosus together with A. naeslundii 19.05. Consideration of the results indicates that each of the sites represents a distinct ecological niche that should be looked at in isolation, and that the following parameters may be important in relating the bacterial population of a site to the formation of a carious lesion: (a) qualitative composition; (b) persistence of specific bacteria; (c) concentration of specific bacteria, and (d) the stability of the numerical concentration of specific bacteria.