The cause of neonatal necrotizing enterocolitis (NEC) is unknown. An association between NEC and clostridial infection has been reported from several centers, but the organisms have not been extensively characterized. Clostridium butyricum was isolated from the peritoneal fluid and cerebrospinal fluid of a neonate with NEC. The organism was resistant to the penicillins, but sensitive to vancomycin. Toxin production was demonstrated. Although the role of clostridial toxins in the pathogenesis of NEC is unknown, clostridial toxins are well established as the causes of two other intestinal diseases (antibiotic-associated pseudomembranous colitis and pig-bel). Further investigation of the role of clostridia in the pathogenesis of NEC and of the use of oral, nonabsorbable antibiotics in the treatment of NEC is needed.