The present paper deals with parasitic Hexamitidae from Acanthobrama simori, Plagiognathops microlepis, Xenocypris dividi and Xenocypris argentea. A total of 179 specimens of these species of fishes were examined and the following results were obtained: 1 No species of Hexamitidae was found in A. simori, but 4 species in P. microlepis, 16 in X. dividi and 12 in X. argentea. This makes a total of 20 species of parasitic Hexamitidae, among which 7 species are considered new to science. 2. The division of Hexamita nobillis in the hosts might be divided into ten stages, and kinotosomes, flagella and axostyles were formed before the division of nuclei while the chromosomes can not be seen. 3. The ultrastructural studies shoed the rodlike body and the capsula covering the organism and the bacteria inside the organism were present always. The rough endoplasmic reticulum (RER) did not develop in organisms covered rodlike body. 4. The cytochemical studies showed DNA was only rich in two nuclei of flagellates and the main substance of energy storage was glycogen. 5. The culture in vitro showed anarobic enviroments were very important for growth and reproduction of parasitic Hexamitidae. Based on the results, it is considered that the Hexamitidae of fishes are a commensalism, the nutrient substance and the physical and chemicial factors are the main reasons which result for restricting the limited positions of the parasites in the host and that species compositions of the parasites in the hosts may reflect relations among hosts, as shown by analyzing the relations between parasites and hosts. The suitable conditions of reproduction of Hexamitidae in fish host and the route of infection are discussed in this paper and so are the origin and function of the special structures on the pellicle and inside the body of these species. Furthermore, the phylogenesis of 17 species in the genus of Hexamita was preliminarily analyzed using the theory and technique of cladistics.