Members of the Bacteroides fragilis group are among the most common anaerobic bacterial isolates in clinical specimens. Metronidazole, a 5-nitroimidazole, is often used as empirical therapy for anaerobic infections. Susceptibility testing is not routinely performed because of nearly universal susceptibility of Bacteroides spp. to this agent. We report a case of metronidazole-resistant Bacteroides fragilis in the United States and demonstrate the presence of the nimA gene, encoding a nitroreductase previously shown to mediate resistance to 5-nitroimidazole antimicrobial agents in B. fragilis strains from Europe and Africa. Because clinical failures in Bacteroides infections have been associated with the use of inactive antimicrobial agents, clinicians need to be aware of the possibility of metronidazole-resistant B. fragilis strains in the United States and the importance of susceptibility testing in selected situations.