Scanning electron and light microscopy were used in studies of stomach, duodenum, cranial, and caudal portions of the jejunum, ileum, cecum, and spiral colon from 18 gnotobiotic pigs. Five pigs were raised as controls and 13 were exposed at 6 days of age by oral administration of 1.6 X 10(6) colony-forming units of Escherichia coli O138:K81:NM. Infiltration of leukocytes into the mucosa of the stomach seen with the light microscope has not been previously reported. The irregular pattern of the mucosal surface of the stomach formed by the gastric pits and the mucosal extensions on the individual rugae revealed with scanning electron microscopy was different than anticipated. Sections of the ileum from control and infected pigs contained collapsed cells around the extrusion zone at the tips of the villi. These collapsed cells were more numerous in infected pigs and appeared to have sloughed from the area of the extrusion zone resulting in exposure of the lamina propria. Cecum and spinal colon were free of changes. Alterations of the mucosa of the intestinal tract of gnotobiotic pigs infected with E coli as visualized by scanning electron microscopy were considered too inconsistent to be of diagnostic significance.