Ectopic thyroid tissue was identified at the base of the tongue in a 9-year-old, euthyroid, domestic shorthair cat. The lingual submucosa and the muscles were replaced by thyroid tissue, which consisted of various-sized follicles each lined by a row of cuboidal epithelium. Many follicles contained eosinophilic, periodic acid Schiff-positive, colloid material. Imunohistochemically, the epithelial cells stained positive for AE 1 /AE 3 and thyroglobulin. No calcitonin positive cells or parathyroid tissue was seen. The cat has been followed for 1018 days after surgery, is clinically well and remains euthyroid.