C cells of the thyroid gland of 85 European bison, 43 males and 42 females, of age ranging from 1 month to 25 years were examined. For the identification of the C cells by Grimelius silver impregnation, toluidine blue, pseudoisocyanin and azur A stainings were employed. In the thyroid of the bison the C cells in the central region were distributed denser than in the peripheral region. They may particularly appear in great numbers in the interior of ultimobranchial (UB) remnants; in these cases, however, a great variety of changes has been observed. In part of the adults animals individual differences of the hyperplasia C cells in the preserved UB structures were observed: C cells have not been detected in the internal parathyroid glands. In the discussion, the meaning of paracrine has been described in autoregulatory mechanisms controlling the thyroid activity.