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Medial fragments of the left thyroid lobe were examined in 84 European bison, Bison bonasus (Linnaeus, 1758). In 60 of the examined thyroids unusual follicles recognized as the follicles of ultimobranchial origin were noticed. In 9 cases they formed so-colled "preserved" ultimobranchial bodies clearly isolated from the thyroid structure. C cells were found in the folicles of the ultimobranchial bodies mainly in a typical parafollicular position, like that in the proper thyroid gland.