Amyloodinium sp. was found on the gills of mullet (Mugil cephalus) cultured on land. No external symptoms in the diseased fish were found except decoloration of the gills. In fresh preparations of the gills the parasites were opaque round or oval shape with a bright nucleus and 43.5 ㎛ (18.2～72.7, n=20) in size. In preparations added a drop of Lugol solution, they were black with the same shapes in fresh preparations and 43.5 ㎛ (n=20) in size. The parasites were stained black and blue in a droplet of Lugol solution and Diff-Quick III solution, respectively and their sizes were a little larger than in wet preparations. After stained with May-Grunwald Giemsa, the parasites appeared granular eosinophlic in the peripheral cytoplasm and granular strong basophilic in the center. In silver impregnated specimens the peripheral granules were negative and the central ones positive. The granules appeared brown in purplish cytoplasm after staining with Lugol solution. The parasites developed by binary division when they were cultivated in filtered seawater at 20℃. Histopathologically severe epithelial hyperplasia and fusion in the gill filaments resulted in clubbing, especially the proximal region of the filament. Epithelial hyperplasia was also found in the basal regions of the gill filaments and some epithelial cells were occasionally detached from the filaments. Some pear-shaped trophonts of the parasites with rhizoid attached on the gill filaments showing hyperplasia of the epithelial cells and mucous cells.