The tegument of the procercoid of Bothriocephalus acheilognathi was studied by transmission electron microscopy (TEM) The tegument of the procercoid consists of an outer syncytial layer with underlaying perikarya. The outer portion of the tegument is composed of a dense layer of microtriches and some tumuli. The microtriches are longer and thicker than those on the adult worm, which are monomorphic and might be stripped off during their establishment in definite hosts and then re-occur on the tegument surface of adult worms. The tumuli are spaced on the anterior part of the procercoid, containing electron-dense inclusions. Three types of secretion processes are observed in the tegumental cytoplasm, i. e., apocrine, eccrine and microapocrine processes. The ducts of the penetration gland open at the hook region at the posterior end of the procercoid. Muscle bundles can be seen within the perinuclear region and just below the fibrous layer. The function of tumuli of procercoid also is discussed in this paper.