In the present work experiments were carried out to investigate the hazardous effects of the organophosphorous insecticide (Hostathion) on the histology, histochemistry and physiology of the liver of the catfish (Clarias gariepinus) after exposure to LC50, VzLCso and ViLCso of the insecticide for different periods. The results showed that the histopathological changes induced in the liver were mainly represented by liver cord disarray, cytoplasmic vacuolization of the hepatocytes, damage of blood sinusoids, blood vessel congestion and inflammatory leucocytic infiltration. The histochemical observation revealed marked reduction in glycogen contents and total protein contents of the liver cells as compared with the control materiaL Physiologically, different treatments at different time intervals resulted in variable degrees of inhibition in 5'-nucleotidase and alkaline phosphatase activities. These changes were both concentration- and time- dependent.