Abstract: It is now generally accepted that only the gastric glands of mammals have separte acid-producing parietal cells and zynogenic chief cells; in fishes, amphibians and birds, both hydrochloric acid and pepsinogen are assumed to be secreted by one type of cells-the oxynticopeptic cells. The present observations on the microscopic and ultramicroscopic structure of the gastric gland cells of Tilapia(Orochromi niloticus)indicate that these cells are different from the oxynticopeptic cells but are very similar to the parietal cells of mammals except that they are deficient in intracelluar secretory canaliculi. They contain full tubulovesicular systems and mitochondria, but have not pepsinogenic granules, and very few rough-surfaced endoplastic recticula(RER). The gastric gland cells of another group of fishes, including Channa argus, Silurus asotus and Pelteobagrus fulvidraco etc., are typically oxynticopeptic cells, which have abundant pepsinogenic granules and RER. When the Western method of Prussian blue reaction for the in vivo demonstration of HCl secretion was used, it was shown that the two kinds of gastric cells described above both have a function of HCl secretion. However, the Adams method for the demonstration of tryptophane failed to deteote pepsinogen granules in either gland cell, probably because the tryptophan content in the pepsinogen of Channa argus is too low to be detected.Owing to the distinct difference in natural diet of the two groups of fishes, the structure of their alimentary canals and the structure and function of gastric gland cells are apparently different.The tubulovesicular systems, which is the basic structure for HCl secretion, exists in both groups of fishes. It is composed of well-developed short tubules and vesicles. There are probably two forms of acid secretion is fish, i.e. aprocrine and merocrine. During acid secretion, the transformation between apical plasma membrane, tubulovesicular smooth membranes and golgi membranes is velated to the form of acid secretion.