Aspirates of pus from perirectal abscesses in 28 children were studied for aerobic and anaerobic bacteria. A total of 87 isolates (64 anaerobic and 23 aerobic) were recovered from the patients, an average of 2.3 anaerobes and 0.8 aerobes per specimen. Anaerobic organisms alone were recovered from 15 specimens (54%), and in nine specimens (32%) they were mixed with aerobic organisms. Aerobic organisms were recovered in pure culture in only four patients (14%). The predominant anaerobic organisms were Bacteroides sp (32 isolates, including 14 B fragilis group and seven B melaninogenicus group), Gram-positive anaerobic cocci (15), Fusobacterium sp (six), and Clostridium sp (three). The predominant aerobic organisms were Escherichia coli and Staphylococcus aureus (six of each), group A beta-hemolytic streptococci, Pseudomonas aeruginosa, and Proteus morganii (two of each). Five children had a serious underlying chronic disease; three of these had an accompanying bacteremia and two died. Incision and drainage were performed on all patients: some children also received parenteral, or oral antibiotic treatment, or both.