Clostridium botulinum organisms and toxin were identified in the feces of six infants, aged 5 to 20 weeks, who had illnesses clinically consistent with botulism. Five of the infants lived in California and became ill within a six-month period in 1976; one infant became ill in New Jersey in 1975. Three cases were type A botulism, and three were type B. No source of ingested botulinal toxin could be found in any case. However, one infant with type B botulism had ingested a food containing C botulinum type B organisms, but no toxin was found in it. The clinical findings in these cases include constipation, weak sucking and crying ability, pooled oral secretions, cranial nerve deficits, generalized weakness, and, on occasion, sudden apnea. A characteristic electromyographic pattern termed "brief, small, abundant, motor-unit action potentials" (BSAP) was observed. The sources of C botulinum toxin for these six infants is thought to have been in vivo (gastrointestinal) production following ingestion of C botulinum organisms. Studies are underway to determine the full clinical spectrum, incidence, and potential public health importance of this infectious disease newly recognized in infants.