The in vitro activity of ciprofloxacin against 30 Bacteroides fragilis group, 30 Clostridium, and 30 Peptococcaceae strains was determined by the agar dilution method in two different culture media at three pH and in three inoculum densities. In Wilkins-Chalgren agar the MICs for 90% of the strains were 0.12 micrograms/ml for B. fragilis, 0.5 micrograms/ml for Clostridium spp., and 2 micrograms/ml for Peptococcaceae. The pH and medium composition affected the MICs of ciprofloxacin.