Abstract: Epistylis eriocheiri Li sp.nov,was isolated from the pincers and legs of Eriocheir sinensis,which were collected from the estuary of Tangxun Lake,Wuhan,China,in September,2008.Living observation and protargol-impregnated technique as well as scanning electron microscopy (SEM) and transmission electron microscopy (TEM)were used to describe systematically the morphology and infraciliature system of these freshwater ciliates,especially their fine structures.As an epibiotic and freshwater species,Epistylis eriocheiri lived in a colony composing of about 100-150 zooids,and it had a dichotomously branch stalk,outside which surrounded a thin layer of shreath.Its whole body was relatively soft and half-transparent; when extending completely,the zooid possessed long ovoid in shape,about (110-170) μm×(50-100) μm in vivo,with ratio about 2：1 of length to width.The organism of the zooid could not be found in the lower 1/3 of the ciliate,thus it looking like spacious and half-transparent; the other 2/3 of the cell,however,contained most of the organisms,such as cytopharynx,contractale vacuole,macronucleus and food vacuole; the cytopharynx extended to the upper 2/3 of the cell,and three shaking peniculus could be seen inside it;contactale vacuole was the only one,about 15μm in diameter,and obliquely locates below peristomial lip; the macronucleus was horse-shoe shape,located horizontally and lied below peristomial disc.The infraciliature system was composed of three kinds of fibers: peristomial lip fibers (PLF),longitudinal fibers (LF) and circular fibers (CF).PLF,which was a close circular structure,located on the edge of peristomial disc,and interconnected with CF and LF to form a single fiber system that might provide the mechanism by which the zooid contracted and relaxed; upward PLF located many thin and dense CF,which connected with each other by some little fibers.LF spread through the whole body; the former LF interconnected with PLF to form a dense net-like structure; the middle LF,which was slender,spindle,regularly and densely arranged,extends to aboral wreath of the cilia (AWC); the latter LF was rather robust,and interconnected with each other to form “Y”-shape fiber,which assembled with each other to terminate in the scopula.Through SEM,it was found that overview of the zooid looked like a barrel,and the stalk was rather slender and straight; the cilia lied in the basal peristomial disc was barrel-shaped,but the cilia on the top of the cytostome (Cy) was petal-like shape; the grooves and ridges on the surface were rather obvious,which interconnected with each other to form chain-like structure; the stria,about 0.5μm in width,were wave-shape and regularly arranged on the surface.Through TEM,it was observed that the pellicular layer was tooth-like shape,and included many structures such as ridge-like protuberance (RP),microtubule between pellicular alveoluses (MP) and pellicular pore (PP); the protoplasm layer included two parts: one part was dense protoplasm layer,which was just below pellicular layer,and had not any structure except some microtubules; the other was loose protoplasm layer,which included many structures such as mitochondria (Mit),macronucleus (Ma) and food vacuole (FV).