A monoclonal antibody-based immunoassay for localization of tetrodotoxin (TTX) in the skin of a brackishwater puffer Tetraodon steindachneri is described in this paper. TTX was recognized in the undifferentiated basal cells and succiform cells in the skin under light microscope. Malpighian cells of the skin did not exhibit any TTX antigen. Neither gland nor enclosed gland-like apparatus possessing TTX was apparent in the skin.