In the routine bacteriological examination of 8,423 clinical specimens, 906 strains of anaerobic bacteria were isolated at National Taiwan University Hospital between April 1972 and April 1976. Most of the strains were isolated from sites allied to the abdominal cavity and genito-urinary tract. Bacteroides fragilis was the most commonly isolated pathogen. The minimal inhibitory concentrations of 10 different antimicrobial agents to 74 clinical isolates of B. fragilis were determined by an agar dilution method under anaerobic conditions. The percentages of the strains susceptible to clindamycin, carbenicillin, ampicillin, cephalothin, and tetracycline were 98, 92, 75, 41 and 5%, respectively. Rifampin, metronidazole and chloramphenicol also inhibited all isolates in clinically attainable concentrations. All strains were resistant to the aminoglycoside antibiotics, kanamycin and gentamicin.