Indian major carp (Labeo rohita) were exposed to 1/10 and 1/5 sub -lethal doses of hexachlorocyclohexane during a 45-day trial period. Fish were sacrificed at the end of the trial period to study light microscopic changes associated with toxicity. Organ tissues, viz., liver, kidney, gill, skin, muscle, heart and brain were examined for histopathological study. Swelling of the hepatocytes with diffuse necrosis and marked swelling of blood vessels were observed in the liver tissue. Tubules of the kidney were distended, with tubular cells of posterior kidney exhibiting marked necrotic changes. Gill tissue showed fusion of primary lamellae, congestion of blood vessels and hyperplasia of branchial plates. Pericardial sac was moderately thickened and extensively infiltrated with leucocytes. Marked neuronal cell degeneration with loss of Nissl substances and microgial nodules could be observed at the cerebrum.