The antimicrobial susceptibility of clinical isolates of Bacteroides fragilis and Bacteroides thetaiotaomicron collected from December 1990 through November 1992 was determined by the agar dilution technique. Metronidazole, imipenem, and cefoperazone/sulbactam remained highly active against both organisms. Rates of resistance to those agents were 0, 2%, and 0.9% in B. fragilis and 0, 0.9%, and 3% in B. thetaiotaomicron, respectively. Cefoxitin and other cephamycins were active against B. fragilis; rates of resistance to these agents did not tend to increase. With the inclusion of these data, the variation of rates of resistance to several agents was summarized for each year from 1987 to 1992. Rates of resistance to imipenem decreased in 1991 and 1992 among isolates of B. fragilis (2.3% in 1991, 1.8% in 1992) and B. thetaiotaomicron (2.4% in 1991, 0 in 1992). Rates of resistance to cefoxitin in B. thetaiotaomicron varied from 10% to 38% during these 6 years, though the distributional peak of MIC values did not change. The rate of resistance to ofloxacin in B. fragilis increased from 42% in 1989 to 81% in 1992. The rate of resistance to ampicillin in B. thetaiotaomicron was 68% in 1992--approximately 30% lower than before. Mostly, however, the rates of resistance to the antimicrobial agents examined did not change significantly.