Clostridium citroniae is a novel species reclassified from C. clostridioforme. Clostridium species are obligate anaerobes and spore-forming gram-positive rods. However, C. citroniae stains gram negative and does not consistently produce spores, making it difficult to identify. We isolated C. citroniae from the blood and peritoneal fluid of one patient, and from the blood of another patient, both of whom were undergoing cancer chemotherapy. (Korean J Clin Microbiol 2010;13:125-127)