A study was made of trends in the susceptibility rates to clindamycin of 338 clinical strains of the Bacteroides fragilis group isolated in the period 1980-83. In 1980, the resistance rate of the species Bact. fragilis to 4 mg/l was only 3.3%, but this percentage increased regularly in 1981 (6.2%), 1982 (15.5%) and reached 19.6% in 1983. Resistance rates in other species of the group (Bact. distasonis, Bact. vulgatus, Bact. thetaiotaomicron) were already high in 1980 and no relevant increase has been documented since then. In order to asses the relationship between clindamycin resistance and hospitalization or macrolides-lincosamides consumption, 184 faecal strains of the Bact. fragilis group isolated from non-treated in-patients, out-patients and healthy volunteers were studied for clindamycin resistance. An unexpectedly high rate of resistance (around 25%) was found in all groups. Resistant strains appeared heterogeneous with regard to phagetype. For unknown reasons, clindamycin resistance seems to be widespread among intestinal Bacteroides strains of the human population, at least in our region.