Abstract: Morphological changes on the filaments and lamella on gills of Maccullochella peelii infected by Ichthyophthirius multifiliis were observed under a scanning electron microscope. The invaded I.multifiliis could be found beneath the flat epithelial cells, on surface of the filaments, as well as in between the lamella of the gills. After being infected with the disease, the filaments and lamella on the gills began to swell and deform, the ossicles on the filaments became exposed and deformed, the lamella thickened, gaps narrowed, apical hyperemia enlarged with blood-filled spherical or rod-shaped tips, and the cellular mucous and chloride cell secretion increased.