This is a case report of an endodermal sinus tumor arising in the stomach of a 72‐year‐old Japanese woman. The tumor showed Borrmann 2 type at the mucosal surface, and the main mass occupied the subserosal layer with outgoing extension. Microscopically, the tumor showed typical characteristics of the endodermal sinus tumor, associated with embryonal carcinoma, poorly differentiated adenocarcinoma and well differentiated tubular adenocarcinoma. Alpha‐fetoprotein was detected in the serum of the patient by radioimmunoassay and within tumor tissue by immunohistochemical techniques. Ultra‐structurally, a characteristic basement membrane‐like material has been identified in intercellular spaces. ACTA PATHOL. JPN. 35: 497–505, 1985.