Morphological characteristics of hemocyte in marine fish louse Argulus caecus were examind. Only a single type of hemocyte, chromophobic granulocyte, was observed in the hemolymph of the fish louse. This cell was round to spindle-shaped one and characterized by the presence of numerous round chromophobic granules (≦ 0.5 μm in diameter) and dull light reddish purple hyaloplasm.