Spontaneous bacterial peritonitis (SBP) rarely presents as pneumoperitoneum. Only four such cases have been reported in the English literature. This report concerns a 65 year-old male patient with SBP and pneumoperitoneum. He has had a history of peptic ulcer. However, upon examination, no associated diseases or compromised immunity was detected. He presented with acute abdominal pain and subphrenic free air. An emergency laparotomy was performed, under the impression of a perforated peptic ulcer. Yet, no intraabdominal pathology except 200 ml of purulent ascites was found. The ascites culture yielded E. coli, B. fragilis, and P. aeruginosa confirming the diagnosis of SBP. Antibiotics which are sensitive to gas-forming bacteria should be prescribed before the result of the ascites culture is known.