Periodontitis is the most common form of periodontal disease initiated and sustained by the composition of microbial dental plaque. The purpose of this investigation was to determine the frequency of Actinobacillus actinomycetemcomitans and.some Gram negative anaerobic rods including Porphyromonas gingivaiis, Prevotella intermedia, and Fusobacterium species in subgingival plaque samples from different forms of periodontitis. The study population comprised 24 subjets, 8 of whom had adult peridontitis (AP) , 9 rapidly progressive periodontitis (R PP) , and 7 juvenile periodontitis (JP). A total of 96 periodnontal sites were examined microbiologically. Subgingival plaque samples were taken from representative affected sites with at least 5 mm pocket depth from each quadrant. Samples were cultured on supplemented blood agar for Gram negative rods and selettive agar (TSBV) for A. actinomycetemcomitans. Total isolates and colony types were determined by examining the surface growth on plates incubated anaerobically for 5- 7 days. All were identified by analysis of colony morphology and pigmentation Gram staining characteristics, biochemical procedures and using api 20 A strips. A , actinomycetemcomitans was not detected in AP patients, whereas it was higher in JP (25%) than in R PP (2.7%) patients. On the other hand, the number of sites infected with P, gingivalis and P. intermedia were higher in AP (34.3%, 40,6% respectively) and R PP (25%, 19.4% respectively) than in JP (14,8%, 14.8% respectively) patients. There were no differences in the frequencies of Fusobacterium s pp. between AP, R PP, and JP groups (53.1 %, 58,3%, 50% respectively). Although A. actinomycetemcomitans. P. gingivallis, and P, intermedia are included in the various types of periodontitis microflora, their incidence and proteins seem to be different, which is of critical importance for disease initiation, progression, treatment and maintenance program.