The incidence of anaerobic bacteria in bile, and the relationship between bacterial species isolated from the bile and the clinical characteristics of disease in these patients was studied. One hundred and twenty-five bile specimens obtained during surgery from the same number of patients were cultured aerobically and anaerobically. Seventy patients (56%) had positive cultures and in 30% of the patients with positive cultures anaerobic bacteria were recovered, usually together with aerobic bacteria. Members of the Bacteroides fragilis group were the most frequently isolated anaerobic organisms. There was a high rate of isolation of anaerobic bacteria from bile in patients with gangrenous cholecystitis (72%) or acute cholangitis (50%). Administration of antibiotics effective against anaerobic bacteria should be considered when treating these diseases.