Thyroid lesions were observed in a series of fish taken from a major exhibit, all of which exhibited flared operculi and rapid gilling. Lesions were processed for light microscopic evaluation. The lesions characteristics ranged from mild follicular hyperplasia suggestive of goiter to "carcinomatous"-like lesions exhibiting marked anaplastic and pleomorphic characteristics. In severe lesions obvious local invasion and possible invasion of blood vessels was seen, suggestive that these lesions possessed characteristics of neoplasia and might have a potential for malignancy. A review of characteristics as related to hyperplastic, benign neoplastic, and malignant neoplastic lesions is provided with a characterization of lesion types as they relate to these categories.