Investigations were carried out to determine a challenge dose of infective tomites of Ichthyopthirius multifiliis which would produce infection in a population of nonimmune channel catfish. The criterion for infection was developed of white spots (mature trophozoites) on 70% of the challenged fish. It was found that exposure of channel catfish to 1,000 tomites per fish and 10 tomites per ml for 30 minutes would consistently produce infection. Reproducible tomite counts could not be obtained using a standard hemocytometer. Using a Sedgwick-Rafter counting cell reproducible counts (no more than 10% difference between counts) could be obtained.