Treatments may only limit mortality by limiting the fungal levels in the water or killing the fungus that has infected the fish. The mechanism for the reduced mortality following formalin treatment is not known. This study demonstrated that formalin reduces mortality associated with S. parasitica infection in rainbow trout with the currently approved doses for external parasites. Although there was no significant difference between the three nonzero treatments, the 100 ppm formalin dose had the overall lowest percent mortality.