The bacteriology of intra-abdominal infections was studied prospectively in 64 patients. Special techniques for isolating anaerobic bacteria were employed along with standard microbiologic methods. Infections included generalized peritonitis, intraperitoneal and retroperitoneal abscesses, as well as abscesses of the liver, spleen, and pancreas. Specimens commonly contained multiple organisms (average, 3.8 per infection). Anaerobic bacteria were isolated most frequently (81% of cases) and multiple anaerobes were the only organisms found in 38% of cases. The remaining isolates were Gram-negative bacilli and Streptococcus species. Antimicrobial therapy of intra-abdominal infections should be directed at Bacteroides fragilis as well as facultative and aerobic organisms that are sensitive to the penicillins, cephalosporins; and aminoglycosides.