Abstract: The present article is a report of the treatment for the control of Sinergasilus yui, a parasitic copepod living on the gills of the Chinese pondfish, Ctenopharyngodon, with a mixture of five parts of copper sulfate and two parts of ferrous sulfate. The effective concentration of the mix- ture is 0.7 p. p. m. in water. The parasitised fish, after being kept in a solution of such a concentration for six days (or an exceptional case, nine days) became free not only from Sinergasilus, but also from parasitic monogenea trematods and epizoic protozoans. The damaged gills of the parasitised fish became also regenerated. This measure of treatment has been extensively practiced.