Histological and ultrastructural studies on the normal structures of hepatopancreas of P. clarkii showed three main cell types, secretory, absorptive and fibrillar cells. The ultrastructural alterations in the hepatopancreas of the red swamp
crayfish P. clarkii exposed to sublethal concentrations of Chlorprifos and neem seed extract (Azadirechtein) pesticides were studied. LC 25 of Chlorpyrifos (0.0075 ppm) produced deformation of the apical surface of absorptive cell, dispersedmicrovilli, mitochondrial swelling with ballooned cristae, deformed cisternae of RER, lytic and vacuolated cytoplasm and pyknosis of nuclei. On the other hand, LC 25 of Azadirechtein (4.99 ppm) caused deformation of mitochondria with the sparce of their cisternae, fragmentation of microvilli, ruptured RER beside the presence of
autophagic vacuole containing dark granules and finally bizzar nuclei having segregated nucleoli and irregular clumps of their chromatin. Chlorpyrifos was found to be more toxic than Azadirechtein on P. clarkii.