Abstract: By observations with light and electron microscopy, it was shown that the pronephros of grass carp contained various kinds of immune cells, which can be divided into lymphocytes, plasma cells, granulocytes, monocytes and macrphages. The granulocytes,accoding to shape and size of intracytoplasmic granules, were further classified into three types: type Ⅰ, type Ⅱ, type Ⅲ granulocytes.