The morphology of the mucosal surface of samples from the small intestine of young protein-deficient pigs was examined by scanning electron microscopy. Three pigs had been infected with Ascaris suum for at least 58 days while 1 had remained uninfected; all pigs were housed and maintained under the same conditions, at the same time. The tissue samples collected at post mortem examination from pigs which contained A. suum showed varying degrees of villous atrophy and fusion. In addition, unusual small craters and as yet unidentified objects were observed at the surfaces of the enterocytes of Ascaris-infected pigs. These changes in mucosal morphology were not seen in the tissue taken from the uninfected pig. The possible association between the mucosal lesions and lactose maldigestion in young, protein-deficient pigs infected with A. suum is discussed.