After 9–21 days without oral hygiene eleven experimental subjects with previously excellent oral hygiene and healthy gingivae developed heavy accumulations of plaque and generalized mild gingivitis. The individual rate of development of gingivitis was closely correlated with the rate of plaque accumulation. Characteristic bacteriological changes were revealed in the plaque along the gingival margin during this experiment. Initially, i.e. when the teeth were clean and the gingiva healthy, the extremely sparse plaque flora consisted almost exclusively of gram‐positive cocci and rods. The first phase of plaque development occurred during the first 2 days without oral hygiene and consisted of a proliferation of the gram‐positive cocci and rods and an addition of about 30 per cent gram‐negative cocci and rods. During the second phase (after 1–4 days) fusobacteria and filaments appeared and increased until they each made up about seven per cent of the flora. During the third phase (after 4–9 days) the flora was supplemented with spirilla and spirochetes, and at the end of the period without oral hygiene each of these two groups of organisms accounted for about two per cent of the plaque flora. In specific areas the gingival condition was correlated with the composition of the plaque and it was found that mild gingivitis could be diagnosed clinically at approximately the same time as the complex flora was established. However, sub‐clinical inflammation started much earlier, probably as a reaction to the first phases of plaque development. When oral hygiene was reinstituted, the plaque in most areas disappeared in 1–2 days and after 7–11 days the Plaque Index for each subject was as low as before the experiment. Correspondingly, after 1–2 days most tooth surfaces only harbored the original sparse flora of gram‐positive cocci and rods. The gingival inflammation in an area usually disappeared one day after the pl