Abstract: Ichthyophthirius multifiliis, a widely distributed protozoan ciliate, can infect various aquatic animals and results in Ichthyophthiriasis to cause serious economic losses. In order to clarify the morphology and morphogenesis of I. multifiliis and the pathological injuries of gill caused by this parasite. In present study, we carried on a survey on morphology and morphogenesis of I. multifiliis trophont and gill histopathology caused by this parasite isolated from Culter alburnus. Under light microscopy, trophonts that removed from host tissue assumed a spherical shape with a diameter range from 500 to 800 m; ear-shaped cytostome and bag-shaped cytopharynx located in the anterior field; trophont encysted itself and transformed to tomont at 18 in 1h, and then underwent a rapid sequence of binary fission; a new endocystical membrane formed to separate the daughter cells into two equal groups during 8-cell stage to 16-cell stage. Pathological analysis indicates that I. multifiliis assumed oval in parasitic positions, stimulated and stressed the surrounded epithelial cells, caused deformation of tissue, adhesion of secondary gill and injuries of epithelial cells and even completely damaged the structure of gills. The present study provides detail description of morphology and morphogenesis of I. multifiliis and the mechanism of pathological injuries caused by this parasite, in order to lay the theoretical foundation for clarifying I. multifiliis invasion and development in the host and provide theory for treatment of Ichthyophthiriasis.