Abstract: Microstructure and ultrastructure of the gill of Haliotis diversicolor have been studied by light and electron microscopy. This was done to clarify cytological details of the gill and to provide data for functional considerations of the gill. The results showed that Haliotis diversicolor has bipectinate paired gills. They are located along the fight and left walls of the mantle cavity, the left gill was a bit larger than the right one. Gills are the main organs for respiration in this kind of animal. A gill is composed of one gill axis and numerous gill filaments which showing basic similarities, each gill filament has a blunt free tip and is corrugated in the middle. All filaments are positioned parallel to each other. Three cell types have been distinguished in the epithelium of the gill. There are ciliated cells, cuboidal cells and gland cells. All of the epithelial cells bear microvilli. Ciliated cells are equipped with cilia, mitochondria and Golgi bodies are interspersed in the cytoplasm, most of which are grouped in the apical cytoplasm. Cuboidal cells bear no cilia, and have less mitochondria than ciliated cell＇ s. Four types of gland cells were distinguished, and each type is with a different distribution through the epithelium. Type Ⅰcells with transparent vacuoles, type Ⅱ cells contain electron dense granules, type m cells contain electron-lucent and low electron dense amorphous flocculent materials, type IV ceils contain round vacuoles filled with low electron dense materials. Beneath the epidermis, opaque chitinous rod, muscle fibers connective tissue and hemocoelic space were found. The chitinous rod may form a basal support to keep gill filaments erect. In the connective tissue, there are collagenous fibers, fibrocytes, fibroblasts, fat cells, undifferentiated mesenchymal cells. Blood cells were found in hemocoelic space and some hemocyanin molecule particles, the respiratory protein were found in some connective tissue cells.