Abstract: The present paper deals with 18 species of parasitic crustaceans. Among them, 15 species of copepods, 2 species of Argulus and 1 species of Ichthyoxenus are belonging to nine different genera. They were discovered from 208 individuals of 43 species of freshwater fishes in Lake HongzeHu, Jiangsu province. Among these parasites, 17 species are previously known, only one species is considered to be new to science. Its diagnostic characters are summarized as follows.Ergasilus leiocassi Xu sp. nov. (figs 1—9)Holotype: No. 79005. paratype No. 79008, deposited in Institute of Hydrobiology, Academia Sinica.Host: Leiocassis pratti Gunther.Distribution: Gao Liang Jian Zhen Measurements (mm): Female: Total length 0.78—0.96; cephalothorax length 0.56—0.59, width 0.37—0.42: abdominal length 0.0493—0.0494 (Ist abdominal segment 0.0208, 2nd abdominal segment 0.0182, 3rd abdominal segment 0.0103) and width 0.052—0.054; genital segment length 0.06—0.07, and width 0.09—0.1; caudal rami length 0.0208—0.023, and width 0.018—0.02; egg sac length 0.87—1.05, and width 0.143—0.157.Diagnosis: Body egg-plant-shaped, cephalic segment fused with first thoracic forming cephalothorax, with a triangular area on anterior of dorsal; 4 free thoracic segments, each segment taperes backward; 3 abdominal segments, the third being the narrowest.This new species resembles E. anchoratus in general appearance, but differs from the latter in detail structures.(1) The length of the third abdominal segment in E. leiocassi is only 1/2 as long as the first one, while in E. anchoratus, all abdominal segments are almost equal.(2) Of the new species, the caudal rami are short and stout, and slightly longer than broad, while in E. anchoratus, they are slender, with length about twice as the width.(3) The fifth leg of this new species is short and clubshaped, with two setae, one terminal and the other subterminal, while in E. anchoratus it is papilla-shaped and bears only one seta on its distal end.This new species is also closely related to E. lamellifer, but differs from the latter in (1) lacking the chitinous lamella on second segment of the antenna; (2) Fifth leg having only one segment with two equal setae; (3) With different arrangment of spines and setae in 1st to 4th thoracic legs.The paper also gives analysis in relation to the formation of fauna, reproductive season and distribution of the parasitic crustaceans in the Lake Hongzehu. According to the infection intensity of the parasites, it is suggested that although Sinergasilus and Lernaea polymorpha have done much harms to fishes in other water bodies, they will not cause severe infection to fish in the Lake Hongzehu.