Abstract: 　AIM: To observe the ultrastructural changes in the body wall and the vitelline cells of Paragonimus westermani in vitro and in vivo before and after triclabendazole treatment. METHODS: The worms were obtained from in vitro and in vivo tests. All of the samples were processed by conventional techniques, and observed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). RESULTS: The external plasma membrane and matrix were cracked or disappeared after the treatment. The necrosis of the muscular layer differed. The cell membranes of cortex and vitelline cells were damaged. Nuclear membrane was damaged partially, heterochromatin solidified and condensed to brim and dissolved. The Golgi complex disappeared, endoplasmic reticulum expanded, mitochondria denatured and dissolved. The damage was more serious in vivo than in vitro. CONCLUSION: Triclabendazole is remarkablely effective against Paragonimus westermani by damaging the body wall and vitelline cells, mainly affecting the nuclei, membrane structures and microtubular system.\;

Key words: Paragonimus westermani, triclabendazole, body wall, vitelline cell, ultrastructure