The microstructure and ultrastructure of zebrafish (Danio rerio) spleen were observed by light microscopy and transmission electron microscopy. The results showed that the spleen of zebrafish mainly consisted of splenic pulp and reticular tissue. The splenic pulp was distributed in the whole spleen, which could be divided into white pulp and red pulp. The red and white pulps showed mixture distribution without obvious boundary (Fig. 3). The red pulp accounted for most of the spleen parenchyma, which was mainly composed of dense red blood cells. The splenic sinus was observed, while the splenic cord was not obvious. The white pulp with deeper staining was mainly composed of dense lymphocytes. The nuclei of lymphocytes were stained dark purple. After Gordon-Sweet silver staining to show reticular fibers, splenic nodules could be clearly observed (Fig. 4). Therefore, the red pulp and the white pulp could be clearly distinguished. Splenic nodules were similar with those of mammals, which were mainly composed of dense lymphocytes. Acid phosphatase staining revealed that there were many phagocytic cells in zebrafish spleen (Fig. 4). Electron microscopy showed the red and white pulp ultrastructure of the spleen. There were different types of red blood cells, platelets and plasma cells in the red pulp, and there were lymphocytes, macrophages, monocytes, neutrophils, plasmocytes and reticular cells in the white pulp. The structure of ellipsoid was not observed in the spleen. Multiple vesicles were observed in the lymphocyte cytoplasm, which may be closely related to the immune function of lymphocytes (Fig. 5﹣10). The spleen of zebrafish was similar to that of most bony fish, and the ellipsoid was not observed.