Aggregatibacter actinomycetemcomitans is a nonmotile, gram negative, capnophilic, fermentative coccobacillus, facultatively anaerobic rod first described in 1912 by Klinger as Bacterium actinomycetem comitans. Thismicroorganism colonizes the oral mucous membrane and mineralized surfaces as well as periodontal pockets. There is a strong correlation between loss of attachment of the periodontal tissue and the high proportion of A.actinomycetemcomitans in affected sites. In addition, this species has been extensively studied as a possible causal factor for aggressive periodontitis. A. actinomycetemcomitans has also been implicated in some cases of gingivitis and chronic periodontitis. This presentation aims to show the distribution of Aggregatibacter actinomycetemcomitans in periodontally healthy and diseased Brazilian subjects throughout epidemiological studies conducted by our group recently.