Pus samples of 30 patients with closed dentoalveolar abscesses who had not received antimicrobial therapy for at last two months were screened for the presence of beta-lactamase-producing anaerobic bacteria. From these 30 pus samples, a total of 112 bacterial strains were isolated; 83 of them were strict anaerobes and 29 were aerobes. beta-lactamases activity of the selected anaerobic bacteria was tested after identification of the isolates and was detected in 5 of the total 83 (6%) strict anaerobic isolates, whereas these 5 strains were isolated in 4 of the 30 (13.3%) pus samples. The species with beta-lactamase activity were in the Prevotella intermedia (4 from 14 isolates) and the Fusobacterium nucleatum (1 from 9 isolates) groups. None of the gram-positive and the other gram-negative anaerobic strains were beta-lactamase positive.