Staphylococcus aureus, Haemophilus influenzae, Bacteroides sp. (Bacteroides melaninogenicus, Bacteroides oralis, and Bacteroides fragilis), peptostreptococci and fusobacterium sp. are important pathogens in respiratory tract infections (RTI). These organisms are often recovered mixed with other aerobic, facultative and anaerobic bacteria. A recent increase in numbers of bet-lactamase producing strains of anaerobic gram-negative bacteria in RTI has been associated with increased failure rates of penicillins in eradication of these infections. These infections include chronic otitis media, chronic sinusitis and mastoiditis, chronic recurrent tonsillitis and lung abscesses. The indirect pathogenicity of these organisms is apparent through their ability not only to survive penicillin therapy but also to protect penicillin susceptible pathogens from that drug. These direct and indirect virulence characteristics of anaerobic bacteria require the administration of appropriate antimicrobial therapy directed against all pathogens in mixed infections.