The aerobic and anaerobic microbiology of intra-abdominal infections associated with diverticulitis was studied in 110 specimens from the peritoneal cavity after intestinal perforation and in 22 specimens from abdominal abscesses. Anaerobic bacteria only were isolated from 17 (15%) of the peritoneal specimens, aerobic bacteria only from 12 (11%) and mixed aerobic and anaerobic flora from 81 (74%). A total of 339 bacterial isolates was detected in peritoneal cultures (3.1 per specimen), comprising 155 aerobes (1.4 per specimen) and 184 anaerobes (1.7 per specimen). Anaerobic bacteria only were isolated in 4 (18%) abscesses, aerobes alone in one (5%) and mixed aerobic and anaerobic flora in 17 (77%). A total of 72 bacterial isolates (3.3 per specimen) was detected in abdominal abscesses - 35 aerobes (1.6 per specimen) and 37 aerobes (1.7 per specimen). The predominant aerobic and facultative bacteria in abdominal infections were Escherichia coli and Streptococcus spp. The most frequently isolated anaerobes were Bacteroides spp. (B. fragilis group), Peptostreptococcus, Clostridium and Fusobacterium spp.