Abstract: From January, 1987 to January, 1988,292 specimens of Mystus rnacropterus were collect ed from the Fujiang River at Hechuan county, China, around the middle reaches of the Jiangling River. Through the study of species composition and ecology of the parasitic Protozoa of the fish, the following conclusions can be drawn: Ten species of protozoa were found in the fish, belonging to 4 classes, 7 families and 7 genera. Three of them are new species, i.e. Trypanosoma hemibagri sp. nov., Crytobia cheni sp. nov. and Henneguya hemibagri sp, nov., All type specimens are deposited in the Department of Biology, southwest China Teachers University. 1. Trypanosoma hemibagri sp. nov. (Figs. 1—6) The new species is parasitic in the blood of Mystus macropterus. It resembles T. striati Qadri et al., 1955, but differs from the latter in the following aspects. In the new species, form Ⅰ is the smallest in the three forms, and the width of form Ⅲ is larger than that of form Ⅱ. There is no obvious difference in body length between form Ⅱ and form Ⅲ. The flagellum of form Ⅰ is the longest in the three forms. The undulating mebrane of form Ⅰ is the widest in the three forms. The new species has a small blepharoplast. The distance from anterior border of nucleus to anterior extremity of the body is 11.2μm in form Ⅰ, 12.2μm in form Ⅱ , and 11.4 tan in form Ⅲ, much shorter than that in T. striati. 2. Cryptobia cheni sp. nov. (Figs. 7—9) The new species is parasitic in the blood of Mystus macropterus. It resembles Cryptobia (Trypanoplasma) seenghali Wahul, 1985, but differs from the latter in having distinctive longitudinal myonemes and much larger size of the new species. 3. Henneguya hemibagri sp, nov. (Figs. 10—12) The new species is parasitic in the gill raker of Mystus macropterus. It resembles H. vovki Achmerow, 1960, but differs from the latter in the larger size of its light yellow cyst(537—3500μm) and a much thicker sutural ridge on the spore; the two pear-shaped polar capsuties are mostly not parallel and not equal in size. The prevalence is 88.2%for T. hemibagri, and below 34% for the other nine species. The intensity of infestation by the ten species is mild. Statistical analysis shows that corelationship exists between prevalence (P) of some parasitic protozoa and water temperature (T) or total length (L) of the host, i. e, C. cheni: P=0. 7997—0. 0238T, C. cyprini: P=0.79047—0. 02934T, T. nobillis: P=0.2651—0.0082T, P=1. 312976e0.069L, T. domerguei f. latispina: P=—0. 42704+0.07629T—0.0028T2, P=—9.9643+0.8375L, T. parasiluri: P=—1.001+0.170T—0.005T2, T. oinformis: P=—0.3716+0.0811T—0.0025T2, T.hemibagri: p=45.026+3.287L—0.062L2, T. sinesis: P=—2.44+0.49L. There are obvious annual dynamics of prevalence of the parasitic protozoa, except that of T. hemibagri.