The distribution of pig thyroid C cells was demonstrated by a immunoperoxidase method using antiserum to porcine calcitonin. The average number of C cells per 4 mm2 counted in the deep and superficial regions was 127 and 55, respectively. The number in the cranial, middle and caudal regions was 60, 128 and 83, respectively. The number of C cells per follicular and C cells (C cell concentration) in deep and superficial regions was 4.9 and 2.6%. It was 3.2, 4.9 and 3.3% in the cranial, middle and caudal regions, respectively. These results indicate that the number and concentration of C cells were not only larger in deep than superficial regions but also in the middle region than in cranial and caudal in pig thyroid gland. These results also indicate that the C cell distribution in pig thyroid gland is the same as that of dog, human, cat, rat, guinea pig, and mouse.