Fifty-six parathyroid glands from 41 horses were examined with hematoxylin-eosin staining and immunoperoxidase method using rabbit anti-human calcitonin antiserum and an electron microscope. C cells filled with immunoreactive secretory granules and distinguished from nonreactive parathyroid cells were found in 42 cases of the equine parathyroid glands. The morphological characteristics and the sensitivity to the anti-calcitonin antiserum of C cells in the equine parathyroid glands were almost identical with those of C cells in the equine thyroid glands. The parathyroid C cells were much more eosinophilic than the chief cells and the water clear cells in hematoxylin-eosin staining. The ultrastructural features of C cells were apparently different from those of the so-called oxyphil cells. They possessed abundant secretory granules of various sizes and were separated from the stroma by a basement membrane together with other parathyroid cells. Desmosomes were found at sites attached to neighboring cells, and half-desmosomes at sites attached to a basement membrane. No transitional cells between the oxyphil cells and the C cells were observed. This study showed that the immunoreactive C cells in the equine parathyroid glands appeared more frequently in parathyroids fused with thyroids than in ones freed from thyroids. No apparent differences could be observed in the frequency of occurrence of C cells owing to breed, age, and sex of the horses.