A mixed aerobic/anaerobic infection in mice was established by co-injecting Bacteroides fragilis and Escherichia coli into a thigh muscle. Metronidazole administration 6 h after this inoculation resulted in a dose-dependent decrease of Bact. fragilis at 18 h after drug administration. Co-administration of gentamicin with metronidazole resulted in a significant decrease of the numbers of E. coli but did not markedly influence the effect of metronidazole on Bact. fragilis.