Between November 1975--December 1977, in the Laboratory of Microbiology, National Institute of Nutrition, Mexico, 156 cultures for anaerobic bacteria were grown on 118 cases with a clinical picture suggestive of anaerobic infection. There were 42 positive cultures (26.9%) from which 18 (42.8%) grew a mixed flora (aerobicanaerobic). Those 42 cultures came from 35 patients (29.6%) and the 18 mixed cultures came from 18 cases (15.25% global and 51.42% of the cases with anaerobic flora). The highest positive marks were obtained from material provenient from the abdominal cavity, and lesser indexes were obtained from lungs, blood, bone and sinovial cavities. The overall mortality was registered in 24 cases (20.33%) but only in 6 cases (5.8% global and 25% of the total cases) a positive anaerobic culture was obtained. In these 6 cases the cause of death was secondary to the principal disease, and only in one case the patient died from septicemia. The diagnosis of anaerobic infection resides mainly in the clinical findings, secondly in a positive gram stain, and thirdly in the identification of the etiologic agent, that will aid in the choice of a more specific therapy, which must include a surgical excision of the affected tissues.