Leucocyte types involved in tne cellular response of Japanese flounder Paralichthys olivaceus against the monogenean Neoheterobothrium hirame were identified. With electron microscopy, peripheral blood leucocytes (PBL) of Japanese flounder could be classified into four types : lymphocytes, thrombocytes, monocytes and granulocytes. Granulocytes were positive for acid phosphatase and peroxidase activities, and stained with Sudan black B. In fish infected with N. hirame, the ratios of granulocytes and monocytes significantly increased in the PBL composition. Infiltration of monocytes/macrophages and granulocytes, and cells having large electron-dense granules (DGC) was observed in the infection sites. These cells were often observed adhering to the tegument of N. hirame. Monocytes/macrophages showed particularly higher adherent ability to N. hirame than the other leucocytes under in vitro and in vivo conditions. We conclude that monocytes/macrophages, granulocytes and DGC are major effector cells in the defense of the flounder against N. hirame.