Milkfish Chanos chanos fry (22-day old; mean fish total length (FTL)= 13±0.6 mm) were exposed to various concentrations of copper sulfate (CuSO4• 5H2O) to determine its tolerance to the chemical. Results show that milkfish can tolerate CuSO4 up to 29 mg L-1 for 96 h at 29.9 C with a mean survival rate of 95%. Frequent infestation by the parasitic dinoflagellate Amyloodinium ocellatum (Dinoflagellida) on the fins and skin of hatchery-reared milkfish with mass mortalities was observed in a commercial fish hatchery in Oton, Iloilo, Philippines. Treatment with 1, 2, and 3 mg L-1 CuSO4 for 1 and 24 h were conducted to evaluate its efficacy against A. ocellatum on infected 24-to 28-day old milkfish. Although the use of 2 mg L-1 CuSO4 for 1 h or 1 mg L-1 CuSO4 for 24 h was effective in treating A. ocellatum on milkfish, a lower concentration at shorter but repeated exposures may be considered for the complete eradication of A. ocellatum infection.