Anaerobic bacteria often are neglected in discussions of the bacteriology of mediastinitis. Two cases of anaerobic mediastinitis are reported and the literature in this field is reviewed. Anaerobes are important pathogens in the etiology of mediastinitis secondary to perforation of the esophagus, extension of a retropharyngeal abscess, or extension of cellulitis or abscess of dental origin from the neck. Although anaerobes indigenous to the oral cavity predominate in these cases, there are also a few cases reported involving Bacteroides fragilis. The source of the mediastinitis should be considered when antimicrobial therapy is initiated so that appropriate anaerobic coverage can be included when indicated.