The gill monogenean Pseudorhabdosynochus lantauensis has been consistently observed oncultured orange spotted-grouper (Epinephelus coioides) fingerlings. The tolerance level of E. coioides toformalin (H2CO solution) and hydrogen peroxide (H2O2), two of the chemicals most commonly used totreat ectoparasites, was determined. The computed 3h and 6h LC10 of formalin were 322 mg L-1 and 275mg L-1 while the 9h LC10 and LC50 of H2O2 were 255 and 298 mg L-1, respectively, at 28oC. The efficacyof freshwater, hypersalinity, formalin and H2O2 bath treatment against P. lantauensis was determined invivo. Hydrogen peroxide at 200 mg L-1 for 1h was effective in eliminating P. lantauensis on E. coioidesfingerlings with no apparent adverse effect on the host fish.