Primary peritonitis usually refers to a bacterial infection of the peritoneal cavity without a demonstrable intra-abdominal source. Most cases occur in children with ascites resulting from nephrotic syndrome or cirrhosis. Rarely, it may occur in previously healthy children less than 7years of age, usually a girl. Distinguishing primary peritonitis from appendicitis may be impossible in patients without a history of nephrotic syndrome or cirrhosis. Accordingly, the diagnosis of primary peritonitis is made only at laparotomy. We report one case of primary pneumococcal peritonitis in a 27-month-old female who underwent explorative laparotomy to discover the cause of suspicious intestinal perforation and mechanical ileus. Later, pneumococci were cultured in blood and gram-positive diplococci were isolated from the pus of peritoneal cavity.