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Copper sulfate has been used for many years as an algaecide and parasite treatment. The usage of copper has certain issues as there is a thin line that separates effective treatment levels from overdoses. Copper sulfate can be extremely toxic to fish under certain conditions. This study is focused on the effects of copper sulfate on some blood indices in Oncorhynchus mykiss during a 24 hour period. Our research used one procedure 40 Oncorhynchus mykiss with the average weight of 220 ±10 g. The experimental group had 20 fish that were treated with the blue vitriol (dose of 0,012 g/40l), during a 24 hour period. The control group had 20 fish. The presence of copper in the water leads to the significant increase of erythrocyte, hemoglobin, MCHC and leukocyte levels while the MCV levels were noted to be considerably low. Monocytes, unsegmented and segmented granylocites were significantly increased in the experimental group of fish. Lymphocyte count was considerably reduced in the same group of fish.