Classification of peripheral leucocytes in Korean catfish (Silurus asotus) were performed. The leucocytes were classified on the basis of their morphological, cytochemieal and phagocytic characteristics. Four types of leucocytes were observed in the peripheral blood. Neutrophils were round and 12 - $15{mu}m$ in diameter with round, oval or 2 - 3 segmented neuclei. Their cytoplam was stained whitish or grey. Lymphocytes were round and 5.5 - $8{mu}m$ in diameter with large round neuclei which occupied most of the cells. Monocytes were round or irregular cells and 12 - $15{mu}m$ in diameter with bean - shaped or irreular neuclei. Thrombocytes were oval or spindle - shaped cells with oval or spindle - neucle. The cells with greyish cytoplasm ranged 7.2 to $14.4{mu}m$ in length and 5.8 to $7.2{mu}m$ in width. Only neutrophils showed positive reaction in peroxidase and PAS. Neutrophils and monocytes showed active phagocytosis. As the results it is concluded that peripheral leucocytes of Koran catfish could be classified as lymphocytes, neutrophils, monocytes and thrombocytes.