Abstract: The ultrastructure of the tegument of the early excysted P. westermani metacercariae Was observed with transmission electron microscope. The tegument was fine and dense in texture, containing a lot of disk-like bodies and a few mitochondria. The outer-plasma membrane was clear and intact.After the metacercariae were incubated in rabbit anti-serum for 3 hours, metacereanal membrane reaction (MMR) appeared closely to the outer plasma membrane and the tegument was damaged.Indirect peroxidase-labelled immunohistochemical technique was used to identify the location of the MMR at ultrastructural level. The result showed that the antibody-antigen complex was localized on the tegumental surface of the worm. The data constituted the evidence that the MMR resulted from the antibody-antigen reaction and the tegumental surface was the reaction site for the MMR (Figs. 1-4).