SummaryThe presence of calbindin D28K in fish (Heteropneustes fossilis) gill was studied by use of specific antibodies raised against chick duodenal 28 kD a calbindin in immunoperoxidase and electron-microscopic labelling experiments. Immunoreactivity for calbindin D28K, which has been observed in the intestine of a number of avian and mammalian species, is reported for the first time in the gill. It was primarily located in neuroendocrine (NE) cells. Some immunoreactivity was also located in the glycocalyx of the non-endocrine cells, i.e., the pavement cells, which have ultrastructural characteristics quite different from those of endocrine cells.The calbindin-immunopositive NE cells were ascertained in both gill filamental and lamellar epithelium. All the NE cells contained secretory granules as the most distinc tive feature of these cells. Ultrastructurally, two types of NE cells were distinguished according to the morphology of their secretory granules.The calbindin immunoreactivity in the NE cells was stimulated when the calcium concentration of the ambient water was rcduccd. The present findings suggest that NE