The toxic effects of 0.3 ppm (96-hr LC50 value) of mercuric chloride solution on the epidermis of Heteropneustes fossilis at different time intervals have been studied. The epidermis reacts instantaneously by secreting a profuse amount of slime, especially due to hyperactivity of the goblet mucous cells which subsequently degenerate and get lost. Later, all other cell types also get entangled in the degenerative process. The damaged cells, especially the club cells, get sloughed at the surface, forming a thick crust of debris from the degenerating cells. Side-by-side regeneration of the epidermis also continues from the lower layers of the intact tissue with the addition of all the cell types. However, even before attaining maturity, these budding cells, especially the club cells, show signs of degeneration, their degeneration occurring faster than their regeneration. Thus most of the space left behind by the degenerated club cells gets quickly filled with haphazardly arranged polygonal epithelial cells, giving the epidermis an altogether different morphology.