The seasonal fluctuation of the protozoan parasites (lchthyophthirius multifiliis, Ichthyobodo necator and Trichodina spp.) on rainbow trout (Oncorhynchus mykiss), brook trout (Salvelinus fontinalis), and brown trout (Salmo trutta fario) in the Eastern Black Sea Region of Turkey was investigated monthly from April 2005 to September 2006. A total of 4260 fish was examined; the overall prevalence of the I. multifiliis, I. necator and Trichodina spp. infestation in the fish was 20.69, 37.93, and 41.38%, respectively. The most intensity and variety of parasites were observed in May and their infestation continued until October. None of the parasites was observed from October to April. lchthyophthirius multifiliis, and Trichodina spp. are observed on three species of trout and l. necator was not observed on brook trout. After treating fish with different concentration of formaldehyde, trichlorfon, dichlorvos, amitraz, cypermethrin, KMnO4, chloramin-T, salt, and vinegar, salt, formaldehyde, and vinegar were appeared to be the most effective chemicals to treat all parasitic infestation while the other chemicals were not effective or suitable for treatment of all infestation. Concentration of 0.2 ml/l formaldehyde was highly toxic to brown trout.