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Two immunization trials were conducted to evaluate immune responses and host protection of channel catfish Ictalurus punctatus and Nile tilapia,Oreochromis niloticus against Ichthyophthirius multifiliis(Ich)using live theronts and sonicated trophonts.Immunizations with live theronts or sonicated trophonts were done by both bath immersion and intraperitoneal(IP)injection.Cutaneous and serum anti-Ich antibodies were measured 12 and 21 days post immunization for catfish and serum anti-Ich antibodies 12,25 and 180 days post immunization for tilapia.The survivals of catfish and tilapia were determined after theront challenge.Anti-Ich antibodies titers were significantly higher(P ＜0.05)in catfish and tilapia immunized with live theronts by immersion or IP injection or with sonicated trophonts administed by IP injection.In contrast,fish immunized with sonicated trophonts by immersion and with bovine serum albumin(control)by IP injection had significantly lower antibody titers.Immunity was noted only in fish immunized with live theronts by immersion or IP injection or with sonicated trophonts by IP injection.There was a positive correlation between higher levels of antiIch antibodies and host survival in the immunized fish.The results of this study show the vaccination against Ich is an excellent alternative to chemical treatments.