Malignant teratoma of the thyroid gland was found in a 19‐year‐old female. She was given a combined surgical and irradiation treatment. The tumor was 5.0·4.0·3.6 cm and located in the lower right lobe of the thyroid gland. Light and electron microscopical examinations revealed mature or immature derivatives of the three primordial germ layers. The dominant portion of the tumor was composed of numerous masses of cancerous epithelial cells, which were distributed randomly in the mesenchymal tissue. Tumor cells invaded and replaced the surrounding thyroid tissue with occasional tumor thrombi in the lumens of blood vessels. No metastatic tumor was found in the lymph nodes. The patient was given a course of external irradiation, a total dose of 4000 rads, to the right thyroid area. Eight months after surgery, she is well and without any evidence of recurrence or metas