Material from 50 patients with purulent otitis media was investigated. All patients had undergone surgical operation. The results indicate that anaerobic non spore-forming bacteria, especially gram-negative rods of the family Bacteroidaceae, may play an important role in the aetiology of purulent otitis media. In a further study, the pathogenicity of the isolated bacteria was investigated. For this purpose, guinea pigs were challenged by intramuscular injection of the organism. All of the isolated strains were sensitive to clindamycin.