Bacterial species isolated from postoperative infections during the period from July 1982 to June 1993 were studied regarding isolation frequencies of different species in different years in a joint research project involving nine (9) university hospitals in Japan. The obtained results are summarized as follows. (1) Altogether, 1,453 strains were isolated with 153 strains obtained during the most recent one year from July of 1992 through June of 1993. The most numerous source of isolation was postoperative intra-abdominal sepsis. (2) Gram-positive cocci were the most often isolated, among them methicillin-resistant Staphylococcus aureus was isolated in increasing frequencies until June, 1992. (3) In the last one year, Enterococcus spp. was isolated the most from postoperative infections followed by Staphylococcus spp. and Pseudomonas spp. Rates of isolation of anaerobic bacteria were relatively low with annual isolation frequencies ranging 8 to 15 per cent.