Micro-distributions of tetrodotoxin (TTX) in the skin of a brackish-water puffer Tetraodon nigroviridis were investigated by means of a monoclonal anti-TTX antibody under light and transmission electron microscope. In light microscopy TTX antigen was visualized as brown color in undifferentiated basal cells and succiform cells of the skin, while in electron microscopy TTX was detected as black dots of immunogold in lysosomes of basal cells. From the results, it can be inferred that when TTX from the blood plasma of T. nigroviridis enters into the undifferentiated basal cells through diffusion, it is taken to the lysosomes by phagocytosis and exists there through binding with internal constituent(s) of the organelles.