The population of the copepod, Ergasilus anchoratus Markewitsch parasitic on gills of the Oriental sheatfish, Silurus asotus, was investigated in Jiangkou reservoir of Jiangxi province. Prevalence and abundance of the copepod exhibited a pattern of seasonal changes with higher levels observed in early summer. Reproduction of the copepod was also seasonal as evidenced by the higher percentage of gravid copepods in early summer and lower in autumn and winter. The proportional distribution of E. anchoratus was found on gills of S. asotus with the middle gill arches having the majority of the parasites. The preference. was to some extent related with the population abundance: at higher abundance e. g in early summer more parasites were found on the two middle gill arches. Further investigations on the microhabitat of E. anchoratus in presence of other species of parasites are certainly needed to clarify the gillarch preference by the copepod.