The toxicity of 4000 ppm (96 h LC50 value) of the inorganic fertilizer ammonium sulphate on the epidermis of Heteropneustes fossilis (H. fossilis) at different intervals of time has been studied. The destruction induced by the ammonium salt is massive. Secretion of a copious amount of slime from the goblet mucous cells leading to the exhaustion and/or shedding and subsequent disappearance is perhaps the first reaction to the toxicity of the irritant. Later, the polygonal epithelial cells of the outermost layer show cyclic stages of necrosis and sloughing followed by their regeneration and repair. The contents of the club cells show enormous shrinkage and condensation with subsequent replacement of their areas with a hazy substance. In the earlier stages of exposure, regeneration takes place quickly, side by side with the degenerative changes at different stages of experimentation. Later after 8 to 10 d, the degenerative changes slow down and the epidermis appears identical to that of the normal skin. Goblet mucous cells also showed several cyclic increases followed by decreases in number and activity.