The susceptibility and inoculum effect of 190 selected Bacteroides fragilis group isolates were determined with six beta-lactam antibiotics by two different methods: agar dilution and broth microdilution. The results were analysed by species within this group of pathogens. The evaluation showed that imipenem had superior activity and was very stable to increased inoculum concentration. Cefoxitin and piperacillin exhibited very good activity and very small inoculum effect; latamoxef showed stability to changes in the inoculum concentration but only intermediate activity; while ceftizoxime and cefoperazone exhibited a marked inoculum effect and relatively poor activity. Resistance among the species was as follows: B. distasonis greater than B. thetaiotaomicron greater than B. ovatus greater than B. fragilis. The inoculum effect was higher when tests were performed in broth rather than in agar. In general, the effect was more pronounced among B. ovatus and less among B. distasonis isolates.