The morphology of the duodenal mucosal surface of chickens infected with Eimeria acervulina has been studied on days 5, 6, 8 and 11 post-infection and compared with that of the normal duodenal mucosa using the scanning electron microscope. The normal duodenum possessed tall, spatulate villi with horizontally-arranged surface clefts upon which a regular pattern of hexagonal absorptive cells and goblet cell mouths was superimposed. A maximum degree of villous atrophy was shown on day 5 post-infection, the villi being thickened and flattened with frequent adhesions and fusions between them. The general effects of the disease together with the physical presence of intracellular parasites caused considerable changes in size and shape of the cell apices. By day 8 the mucosa was clearly recovering, with a normal pattern of somewhat thickened and truncated villi beginning to re-appear and by day 11 post-infection the mucosa had regained an outwardly normal morphological pattern. The mucosal changes are discussed in relation to similar changes which occur in diseases of the alimentary tract of other animals, including man.