The aim of the present study was to evaluate the effect of a strict supragingival plaque control regimen on bacterial repopulation following scaling and root planing. 7 patients with moderate to severe inflammatory periodontal disease received a full-mouth scaling and subgingival curettage. Using a split-mouth design, 2 sites of opposite quadrants were submitted to professional supragingival plaque control 3 X a week while the contralateral sites served as controls. Clinical and bacterial examination were performed on days 7, 14, 28, 49, 56, 63 and 70 following therapy. All clinical parameters (P1I, GI, probing depth, attachment levels) showed significant improvement after scaling and root planing. Shifts in the subgingival bacterial population observed by dark field were also noticed following curettage: there was a decrease in the proportions of spirochetes and motile rods and an increase in that of coccoid cells. However, bacterial distribution tended to return to base line values towards the end of the observation period. No difference in the pattern of bacterial recolonization of the subgingival area could be detected between the sites under strict supragingival plaque control and the control sites.