Abstract The acute toxic effect of BHC on gills, liver, stomach and intestine of a fresh water teleost, Trichogaster fasciatus, has shown pathological changes after exposure to a sublethal (1.5 ppm) and a lethal (3.6 ppm) concentration of the toxicant for 24 and 96 h. Damage to the different tissues was more pronounced in a lethal concentration of the chemical and increased time of treatment. The gills exhibited greater damage compared with the liver, stomach and intestine. Swelling of the secondary lamellae leading to the detachment of the epithelium from the pillar-cell system (PS) and its partial degeneration with loss of rigidity of the PS and hypertrophy in the lamellar channels were the major changes induced. In the liver, the tissue showed prominent splitting (SP) due to the widening of the sinusoids. The cells lost their shape, the membrane was ruptured, the cytoplasm became granulated and necrosis was evident. Degeneration of the epithelium (EP), shrinkage and atrophy in the tunica propria (TUP) and vacuolation (VA) in different layers of the stomach and intestine were also observed.