The morphological characteristics of the minor duodenal papilla were studied histologically using surgically resected specimens according to the presence or absence of an opening of the accessory pancreatic duct in 30 patients (22 with a ductal opening and 8 without such an opening) who had undergone pancreatoduodenectomy within the past 4 years. The following results were found: 1) The minor duodenal papilla was composed fundamentally of the accessory pancreatic duct, pancreatic tissue of the dorsal pancreas which penetrated the duodenal proper muscular tunics and the surrounding fibrous connective tissue. 2) In those specimens with a ductal opening, smooth muscle fiber bundles derived from the duodenal proper muscular tunics surrounded the accessory pancreatic duct and seemed to possess sphincter action similar to that of Oddi sphincter muscles. 3) In those specimens without a ductal opening, the accessory pancreatic duct terminated blindly in the minor duodenal papilla which was comprised mostly of pancreatic tissue and seemed to have no sphincter muscles. 4) Islet cells in the pancreatic tissue of the minor duodenal papilla were rich in B-cells which were round to ovoid in shape and sharply outlined.