White spot disease, caused by the protozoan parasite Ichthyophthirius multifiliis (I. multifiliis), invades nearly all fresh water fish species and causes huge economic losses. In Germany no protocide substance is legal for the treatment of I. multifilis. As an alternative substance the peracetic acid (PAA) was tested to treat the free invasive stage (theront) of the parasite. PAA concentrations of 0.3 ppm were able to kill all theronts in 120 min in our investigations. As a result of these investigations we recommend an interval-application of 0.3 to 0.5 ppm PAA for 30 to 150 min. This application should be prolonged for two life cycles of the parasite. Biotic parameters as e. g. fish species, and age as well as abiotic parameters as e. g. temperature, pH and organic load of the water could possibly influence the efficiency of the PAA application and should therefore be taken into account while picking the dosage and length of the PAA exposure.