Forty-six adult periodontal patients, selected on the basis of clinical examination, and 46 adult healthy subjects were examined. The subgingival plaque samples from one inflammatory and one non-inflammatory site of each periodontal patient were studied to determine Porphyromonas gingivalis prevalence related to other periodontal micro-organisms and to periodontal tissue destruction. The results showed Porphyromonas gingivalis as the main pathogenic micro-organism isolated in the inflammatory sites together with Bacteroides forsythus. Peptostreptococcus sp., Actinomyces sp. and Prevotella sp. were found as a normal oral flora in the healthy subjects. Fusobacterium nucleatum, Prevotella intermedia, Campylobacter rectus and Eikenella corrodens were detected both in inflammatory and in non-inflammatory sites of periodontal patients as well as in the healthy subjects.