In a bacteriologic investigation of infants with necrotizing enterocolitis (NEC), 16 of 50 infants had clostridia in cultures of blood or of peritoneal fluid obtained by paracentesis. Twenty-eight of the 50 infants had enteric bacteria other than clostridia, and six infants had sterile cultures. Of the 16 infants with clostridia, nine had C. perfringens and seven had other species of clostridia. Compared to infants with nonclostridial NEC, those with clostridial NEC were larger and more mature, had more extensive pneumatosis intestinalis and gangrene and more rapid progression of NEC. The nine infants with C. perfringens had a fulminant form of NEC, analogous to gas-gangrene of the intestine. Mortality in this group was 78% (7/9). The seven infants with clostridial species other than C. perfringens had a mortality comparable to that of infants with nonclostridial NEC (32%). Improved survival from NEC associated with C. perfringens may be possible only by prevention, rather than earlier diagnosis and improved heroic treatment.