Abstract: The present paper embodies the results of studies on the protozoan parasites from 389 individuals of Aristichthys nobillis and Hypophthalmichthys molithrix.Apart from the species of Myxosporidia 20 species have been re-corded,among them,14 species are previously known,namely,Euglenosoma branchialis Davis,Cryptobia branchialis Nie,Costia necatrix Henneguy, Glugea hertwigi Weissenberg,Dermocystidium percae Reichenbach-Klinke, Hemiophrys macrostoma Chen,Chilodonella cyprini Moroff,Glaucoma pyriformis Schew.,Ichthyophthirius multifiliis Fouquet,Trichodina pediculus Mueller,T. bulbosa Davis,T. oviformis Chen,Glossatella cylindriformis Chen and Trichophrya sinensis Chen.The remaining 6 species are new to the science,their diagnosis are now summarised as follows: 1.Trypanosoma aristichthysi sp.nov.(Pl.Ⅰ,Figs.5—8)is found from the circulatory system.It has a breadth of 1.2 μ and total length of 31.6 μ The free flagellum is 8.3 μ long.There is a well developed undulating mem- brane which exhibits 5—6 folds.An oval or ellipsoidal nucleus is situated 38% of the total length from the anterior extremity.The cytoplasm contains a few vacuoles.The kinetonucleus is ovoid or ellipsoid in shape,and situated 8% of the body length from posterior end.The blepharoplast is hardly discernible. 2.Cryptobia agitata sp.nov.(Pl.Ⅱ,Figs.9—14)often occurs in enormous numbers on the gill filaments or body surface of the hosts,especially in the case of young fishes.It is triangular or elongated in shape.A blepharoplast is situated at the left side,and about one third of the body length from the anterior extremity.The two flagella,arising from the btepharoplast,are of unequal length.One is anteriorly directed and about the same length as the body,while the posteriorly directed one,which may be attached to the surface of the body for a short distance,is about 1.5 of the length of the body.A rounded nucleus is located near the right side of the middle of the body and contains spherical granules of chromatin,attached to the nuclear membrane. A club-shaped kinetonucleus,which is located at the front of the nucleus, is especially characteristic to this species.The body measures from 4.6 to 7.7 μ(average 6.7μ)in length and 3.2 to 4.6μ in width(average 4.1μ). 3.Eimeria sinensis sp.nov.(Pl.Ⅱ,Figs.15—20)has a spherical oocyst measuring 10.4μ(9.2—10.7μ)in diameter,and with a thick,smooth and transparent wall.The cytoplasm has a coarsely granulated appearance. In mature oocyst,the spore is ellipsoidal in shape and contains two club-shaped sporozoites and a large spore residual body.No oocystic residual body and micropyle are present.There is one small,dark,irregularly shaped polar inclusion. 4.Hemiophrys disciformis sp.nov.(Pls.Ⅲ,Ⅳ,Figs.21—28)was found both on gill filaments and gill rockers.It occurres normally in an enclosed condition or cyst,which appears to be continous with the cuticular membrane of the gill epithellium of the host.Within the cyst the parasite rotates actively.It is round or ovoid in shape.The mouth or cytostome is a slipt-like structure and very difficult to be discerned.Two large oval macro- nucleus are always present.Only one micronucleus is observed between the two macronuclei.The contractile vacuoles are small,ten to fifteen in number.The right side of the body is distinctly ciliated,while the left side is deprived of cilia. 5.Trichodina lieni sp.nov.(Pl.Ⅳ,Figs.29—32)is found from the gills of both two hosts.The denticulated ring is well-developed and consists of 21—25 denticles,the usual number being 22.The hooks are sickle-shaped. The rays are slender and sharply pointed at the end.The striated band has 7 to 8 bars between each two denticles,the usual number being 7.Although the macronucleus has the usual horseshoe-shaped outline,the middle region of the "shoe" is often larger than the portions of two arms.A spherical micro- nucleus is located near the end of one arm. 6.Trichodina minuta sp.nov.(Pl.Ⅴ,Figs.33—38)was found from the gills of both two hosts.The denticulated ring contains from 20 to 25 denticles, the usual number being 22 to 24.The hooks are paddle-shaped and the rays are very short or even lacking.The striated band has 3 to 4 bars between each two denticles, the usual number being 4.The macronucleus is not of the usual type of horseshoe-shaped,but it is often in twisted or fragmentary condition.A spindle or ellipsoidal micronucleus is located near the end of one arm. In addition to the above mentioned species,there is an undetermined intestinal flagellate,Hexamitus sp.