Abstract: This article is an account of a new species of trichodinids-Trichodina paranigra sp.nov., a kind of parasites that liveon the gills of Carassius auratus. It gives a detailed description and diagrams illustration of the morphological characteristics including the shape, the size, and all other basic biological features following the widespread acceptance of the uniform specific characteristics proposed by Lom(1960) and the description of denticle elements follows the format recommended by Van As &Basson (1989). It also gives the interspecific comparison with the closely ralated populations.Two new parameters are also added for the diagram of Van et al in this article. Trichodina paranigra sp. nov. (Figure 1- 3;Table 1) Syn:Trichodina nigra Lom,1961 Sensu Chen, 1963 Host and site of infection: Carassius auratus, gills, 2002. Locality:Chongqing, China. Type-specimens: Syntypes on slide(No: 2002- 1210- 01) are deposited in The Key Laboratory of Animal Biology, Chongqing, Chongqing Normal University, China.The morphology of Trichodina paranigra sp. nov. (Measured specimens number: 12):Large, freshwater Trichodina Ehrenberg, 1838. Body diameter 60. 3- 3. 4(56 65um). Diameter of adhesive disc 50. 3- 3. 2(46- 53um). Border membrane 5. 6 0. 5(5- 6um) in width. Diameter of denticulated ring 30. 6 2. 9(27-33- m).Number of denticles 29- 3. 6(25-32).Number of radial pins(mode) per7-8.Span of denticle 16. 0-1. 4(14-17um). Length of denticle 6. 7-1. 7(4-um). Blade broad, almost obliquely rectangular, tapering gradually downwards, length 7. 60. 2(4- 8um). Distal blade surface straight, lower than tangent point. Blade connectionslender.Central part thicker with rounded point fitting tightly into preceding denticle, extending more than half way to Y-1 axis.Width of central part 1. 9-0. 5(1-2um). Length of ray 8. 4- 0. 2(8-9um). Based on methly greenpyronin stained specimens, macronucleus horseshoe-shaped, external diameter 48- m(mode). Distance between terminations of macronucleus 10- m(mode)Micronucleus ellipsoid, situated in + Y position.Adoral ciliary spiral turns 360-400around peristomial disc.