Mucin-producing carcinoma of the thyroid is rare and its histogenesis has been debated. The first case was reported by Diaz-Perez et al. (1976). We herein report the sixth case of mucin-producing carcinoma of the thyroid which was discovered in a 72-year-old Japanese woman. This patient was admitted with a 10-year history of a lump in the right anterior neck, which started to grow suddenly two months before diagnosis. The pathological diagnosis at biopsy was anaplastic carcinoma. A course of radiation therapy followed by chemotherapy resulted in no response. She died of two months after admission. At autopsy, the main tumor was found in the right lobe of the thyroid with metastasis to the lungs and liver. Histologically, the tumor was composed of mucin-secreting glandular cells and nests of epidermoid cells with keratinization and infrequent formation of intercellular bridges. Mucin was demonstrated both intracellulary and extracellulary.