Abstract Successive stages of developing supragingival and subgingival plaque were analysed on days 1, 4, 7, 11 and 14 with dark-field microscopy and anaerobic cultural techniques. One-day-old supragingival and subgingival plaque contained a predominantly Gram-positive coccal flora, with a smaller percentage of Gram-positive rods. There were low proportions of dry spreaders, such as anaerobic Vibrio, Campylobacter, Eikenella corrodens and corroding Bacteroides, wet spreaders such as Capnocytophaga and Selenomonas, and Fusobacterium and Veillonella. Dark-field examination of 1-day-old plaque revealed low proportions of filaments, fusiforms and motile rods. All these subgroups of organisms increased proportionally during a 2 week period at the expense of Gram-positive cocci. Spirochaetes appeared sporadically in 7- and 14-day-old subgingival plaque. Bacteroides melaninogenicus was only found once in 11-day-old supragingival and subgingival plaque. Gram-positive rods were predominant in developing supragingival plaque, whereas motile rods and spirochaetes were found in slightly higher proportions in ageing subgingival plaque. Apart from these minor differences, the composition of supragingival and subgingival plaque during a 2 week period of plaque development was similar. In the upper molar sites, ageing supragingival and subgingival plaque showed a strong proportional increase of dry spreaders, wet spreaders, E. corrodens and Fusobacterium but no shift in the proportion of Veillonella. In contrast plaque on the lower premolars showed a strong proportional increase of Veillonella but no shift in the proportion of Gram-negative rods. These observations indicate discrete site-related differences in plaque development.