Technical Abstract: Ichthyophthirius multifiliisis a protozoan that may infect and significantly damage cultured fish species. One of the purposes of this study was to establish a laboratory shallenge system that could be used to measure the efficacy of various therapeutic agents, such as copper sulfate, in the prevention and treatment of ichthyopthiriosis. Exposure of fingerling channel catfish to at least 2000 tomites of I. multifiliis lead to consistent infections (20 or more trophozoites on the dorsal surface of the head of the fish). Infection was observed in non-copper sulfate-treated contrals at day 5 after exposure with mortality occurring after day 10. In vitro treatment of tomites with various concentrations of copper sulfate indicated a No-Observed Effect Concentration (NOEC) of 0.1 mg/L. The NOEC of copper sulfate for the in vivo co-exposure of channe fish was 0.05 mg/L. To determine the effect of copper sulfate in the treatment of ichthyopthiriosis, fish were exposed to the pathogen for 5 days and subsequently treated with copper sulfate. After 13 daily water changes and copper treatments, trophozoites were completely eradicated in copper concentrations above 0.1 mg/L. Fish treated with 0.4, 0.8, 1.0, and 1.6 mg copoper sulfate/L were free of trophozoites after 5 days of treatment. Concentrations of total suspended solids were inversely