A 78-year-old man with liver cirrhosis was admitted to our hospital because of persistent diarrhea and abdominal pain. Colonoscopy revealed erosions and submucosal tumors in portions of the sigmoid colon and rectum, which were diagnosed as mantle cell lymphoma with biopsy. After chemotherapy consisting of vincristine and prednisone, he developed Aeromonas hydrophila septicemia, resulting in septic shock and necrotizing fasciitis despite prescribing antimicrobial agents to which this organism was susceptible. Whenever we intend to treat patients with hematological malignancies and an underlying chronic liver disease, we must keep this bacterium in mind.