Reports on zonal aganglionosis of the bowel are very rare and the few cases reported concern colon and rectum. We present a case of segmental aganglionosis of the jejunum in a 2-months-old boy. He had to be operated upon twice because of an ileus of the short bowel. At the second operation, a 10 cm long narrow segment of the jejunum was resected. The histological examination revealed a segmental aganglionosis of the plexus myentericus. Possible causes of this rare zonal aganglionosis could be: 1. Local damage of the plexus myentericus through anoxia. 2. Primary abnormity in the gastrointestinal anlage. 3. An intrauterine inflammation. 4. A double migration and development gradient of the precursors of the neuroblasts, from each end to the middle of the gut. A defect in the microenvironment of the enteric neurons would result in their failure in differentiation. This possible explanation would challenge the accepted theory of pathogenesis of colonic aganglionosis.