Clostridia isolated from the feces of ten healthy infants during the first year of life were identified. All infants excreted clostridia during the first week of life; the most common species were Clostridium butyricum, C. paraputrificum, and C. difficile. In the formula-fed infants colonization occurred consistently throughout the year, whereas in some breast-fed infants it was inhibited until weaning. The species isolated most frequently throughout the year were C. paraputrificum, C. butyricum, C. ramosum, C. sartagoformum, and C. perfringens. Of the species often found in infants, C. butyricum, C. difficile, and C. sartagoformum occur rarely in adults, reflecting differences in the environmental conditions in the gut of the two age groups. The mechanism by which environmental conditions select species of clostridia is unknown but may be important to our understanding of a variety of clostridial intestinal infections. It was demonstrated that species of clostridia which have been implicated in the etiology of neonatal necrotizing enterocolitis form part of the normal infant gastrointestinal microflora.