Abstract: Balantidium polyvacuolum Li, 1963, an obligatory intestinal symbiont of Xenocyprinae fish, mainly inhabits in the luminal contents of the middle and hindgut. Though the description and some revisions are given at both light and scanning electronic microscopic levels, the morphological data are still incomplete since fine structural data with taxonomic values are still unavailable. This study examined the ultrastructural anatomy of this Chinese balantidium that inhabits in the intestines of Xenocypris davidi collected from the Jialing River in Sichuan province during fish parasite surveys in August 2011. All fish samples were dissected, and intestines were cut to collect the luminal contents into petridishes for further examination with the aid of a dissecting microscope. The luminal contents of the hindgut containing numerous B. polyvacuolum were selected and fixed directly in 2.5% glutaraldehyde in 0.2 M phosphate buffered saline (PBS, PH 7.4) for 2h at 4℃, then postfixed in 1% (v/v) osmium tetroxide in PBS for 2h at 4℃, this step was followed by a series of dehydration in acetone and embedded in Araldite. Ultrathin sections were cut on a Leica Ultracut R ultramicrotome, stained with uranyl acetate and lead citrate before being observed in a Philips Tecnai G2 20 Transmission Electron Microscope (Netherlands). Its somatic cortex, vestibulum, nuclei, cytoplasmic organelles and the centroventral concave platelet were well described here. Our results indicated that both somatic cortex and vestibulum were composed of pellicle (the outermost cell membranes, pellicular alveoli and the layer of microtubules) and infraciliature (kinetosomes and associated fibrils such as kinetodesmal fibril, postciliary microtubules, transverse microtubules and nematodesmata). However, the nematodesmata of vestibular kinetids were much longer than those of somatic kinetids and usually extended deeply into the endoplasm. As for the nuclei, the macronucleus was club-shaped with several nucleoli and heterochromatin occupying the karyoplasm, whereas the spherical micronucleus was homoeomerous with diffuse chromatin distributed over the karyoplasm. In addition, large amounts of amylopectin granules were observed within cytoplasm of the centroventral concave platelet. We suspected that this region may act as a storage pool of amylose to provide energy for the organism. Besides, the components of the cortex were compared among the related Balantidium species and their possible functions were also discussed.