The prevalence and distribution of the putative periodontal pathogen Eikenella corrodens in the human oral cavity was examined. A total of 508 oral bacterial samples were taken from 10 periodontally healthy adults (PH), 11 adult periodontitis patients (AP), and 6 localized juvenile periodontitis patients (LJP). From each subject, samples of supra- and subgingival plaque were obtained from six to eight teeth as well as samples from buccal mucosa, lateral and dorsal surfaces of tongue, tonsil, and saliva. E. corrodens was cultured from 70% of healthy subjects and 100% of periodontitis patients. Dental plaque appears to be the main oral ecological niche of E. corrodens in PH subjects since it was found in, respectively, 26% and 31% of supra- and subgingival plaque samples and rarely found in other oral sites in these subjects. It was found in 59% of both supra- and subgingival plaque samples from AP subjects, as well as 48% and 64% of supra- and subgingival plaque samples of LJP subjects. In contrast to healthy subjects, E. corrodens was found on the buccal mucosa, tongue, tonsil and in the saliva of patients with periodontitis. The microorganism constituted, on average, 1% to 2% of the total cultivable bacteria in supra- and subgingival plaque samples. The prevalence of E. corrodens in plaque samples was higher in AP and LJP subjects and was significantly different from PH subjects. Within the AP group, the prevalence of E. corrodens in subgingival plaque is significantly higher from sites with GI greater than or equal to 2. These data suggest that E. corrodens is an indigenous oral microorganism which may be an opportunistic pathogen associated with gingival inflammation.