A new myxosporean, Myxobolus episquamalis sp. nov. is described from the scales of the mullet, Mugil cephalus in Japan. Lerge, flat cysts developed on the surface of the apical area of the scales were formed of polysporous trophozoites which varied greatly in shape, being mostly irregularly winding, branched hypha-like bodies. Spores were oval in front view, tapering anteriorly to a blunt apex and lenticular in lateral view. Most spores were completely covered with a thick mucus envelope. Pyriform polar capsules were somewhat unequal in size and without visible filament coils. The sporoplasm contained an iodiophilous vacuole. Each valve had a nipple-shaped thickening with an opening of the polar capsule on the sutural edge at the apex and irregularly folded on the margin. Dimensions (μm) of fresh spores : length, 7.5-9.5 (8.6); width, 6.0-7.5 (6.8); thickness, 4.5-5.5 (5.1); polar capsules : length, 3.8-5.0 (4.4); width, 2.0-3.0 (2.2).