Technical Abstract: The free-living infective theront of Ichthyophthirius multifiliis historically has been thought to be the only stage susceptible to treatment. A technique is introduced to determine the toxicity of compounds to the detached trophont, the settled tomont and the developing tomites within the tomont that emerge as theronts. The toxicity of Wofasteril(c) E400 (40% peracetic acid, PAA) to free-living forms of I. multifiliis was determined shortly after trophonts were physically removed from the surface of the fish and at 2.5 and 24 h after removal. Results indicate that 0.6 mg/L - 0.9 mg/L PAA killed 39 to 82% of the newly released trophonts within 48 h when treated immediately. In a second experiment, trophonts were allowed to settle for 2.5 h after sampling from the skin and then treated for 12 h; concentrations =0.5 mg/L PAA produced significantly less theronts than the controls. In a third experiment, tomonts that were exposed to PAA 24 h after sampling from the skin and treated for 2 or 4 h produced a variable amount of theronts, but the concentrations tested (0.5 – 3.0 mg/L) did not halt theront production. This research demonstrates that encysted I. multifiliis are less susceptible to chemical treatments.