A case report of mucinous adenocarcinoma in anorectal fistula of rectal mucosal origin, though many cases reported were thought to be of anal duct origin. The patient, a 51‐year‐old male, was operated for anal fistula twelve years previously. He presented to us for anal pain, anorectal stricture, and perianal induration. Abdominoperineal resection was performed for proper management. On gross no visible mucosal lesion of anal canal and rectosigmoid colon was revealed. On cross section a gelatinous tumor was found mostly outside the rectal muscle layer, and an internal opening of the lesion was detected in the rectal mucosa. Histologically, the rectal mucosa extended into the edge of the internal opening and the carcinoma appeared just adjacent to this rectal mucosa. Periodate‐borohydride/saponification/PAS stain also indicated that the mucin produced by the carcinoma had the nature of rectal mucosal origin. We considered that the carcinoma of this case originated at the internal opening and developed into the antecedent fistulous track. ACTA PATHOL. JPN. 34: 649–654, 1984.