A 72-year-old woman was admitted with a complaint of a sensation of abdominal fullness. Cytologic examination of ascites revealed many poorly differentiated adenocarcinoma cells. Barium enema study and colonoscopy revealed IIa+IIc-type carcinoma of the descending colon. Endoscopic mucosal resection was performed to determine the histological type and the depth of invasion. The resected tumor was 7 x 6 mm in size, and an amorphous pit pattern was observed in the depressed area by stereomicroscopy. Poorly differentiated adenocarcinoma with signet-ring cells had diffusely infiltrated into the deeper part of the submucosal layer. Immunohistochemical findings showed this tumor to have mucin derived from gastric foveolar epithelium, suggesting that the signet-ring cell carcinoma of the colon showed gastric differentiation. Primary signet-ring cell carcinoma of the colon and rectum is a rare form of adenocarcinoma of the large intestine and shows more malignant biological behavior than ordinary colorectal carcinoma. Early diagnosis and curative operation are important.