Abstract: The ultrastructure of the fibrous sheath,the layer of ganglionic cell bodies and the neuropil of brain ganglion of Oncomelania hupensis was first observed under transmission electron microscope. Ultrastructural changes were found in the brain ganglion after immersing the snails in sodium pentachlorophenate.The mitochondria of nerve fibers and neurons showed denaturation and necrosis,and glycogen in neurones was markedly reduced;the neuropil also exhibited denaturation and necrosis.The ultrastructural findings may explicate in some way the mechanism of molluscicidal action of NaPcP on Oncomelania hupensis via its action on the nervous system(Figs.1-6).