The narrow-spectrum macrocyclic antibiotic fidaxomicin is approved for treatment of Clostridium difficile infection in many countries and is currently under evaluation in Japan for this indication. This study was conducted to evaluate the effects of fidaxomicin and its major metabolite, OP-1118, on Clostridium spp. isolated in Nagasaki University Hospital, Japan. Isolates were cultured and antimicrobial susceptibility analyses performed according to the Clinical Laboratory Standards Institute methods. Ninety-eight isolates were obtained between 2012 and 2015, 50 of C. difficile and 48 of eight other Clostridium spp. Fidaxomicin had the lowest minimum inhibitory concentration (MIC) of the antimicrobials tested against C. difficile, with MIC90 (MIC range) 0.12 μg/mL (0.015-0.25), versus vancomycin MIC90 0.5 μg/mL (0.5), metronidazole MIC90 0.5 μg/mL (0.12-0.5), and OP-1118 MIC90 4.0 μg/mL (0.5-4.0). Fidaxomicin and OP-1118 each had a similar spectrum of activity against the other Clostridium spp. C. butyricum and the 29 fidaxomicin- and OP-1118-susceptible C. perfringens isolates had the lowest MIC values, and C. bolteae and C. hathewayi higher. All the C. ramosum isolates (n = 6) and one of 30 C. perfringens isolates had low susceptibility to fidaxomicin and OP-1118 (i.e., MIC >64 μg/mL). In summary, this study showed that fidaxomicin was active against a number of Clostridium spp., including C. difficile. Fidaxomicin was generally more effective than its major metabolite OP-1118, but both showed a similar spectrum of activity, suggesting that OP-1118 contributes to the antimicrobial activity of fidaxomicin. These findings were broadly in accordance with those of similar studies conducted in other settings.