We report a suspicious case of spontaneous bacterial peritonitis (SBP) caused by Providencia rettgeri and Clostridium perfringens in a patient with alcoholic cirrhosis. The patient presented with altered mentality and was taken to the emergency room. He was diagnosed with SBP after abdominal paracentesis and computed tomography and was treated with ceftriaxone and metronidazole. The pathogens were identified under suspicion of polymicrobial infection because of Gram-staining discrepancies between broth from blood culture bottles and colonies on solid media. He died of septic shock despite transfer to the intensive care unit. Although we could not conclude which organism had the leading role in this case of SBP and septicemia, we did verify the importance of Gram staining in a microbiology laboratory in terms of quality assurance.