Abstract: Salmo trutta fario were exposed to a concentration of cobalt chloride (180 μg L-1 of) at 12 h intervals, for 28 days in order to determine the effects of this chemical compound on the hematological parameters of this fish. The blood samples were obtained from the all control and treatment fish at the end of the exposure period. Increases were observed in Red Blood Cell (RBC), thrombocyte count, Erythrocyte-Sedimentation Rate (ESR) and hematocrit. On the other hand White Blood Cell (WBC), hemoglobin, Mean Corpuscular Volume (MCV), Mean Corpuscular Hemoglobin (MCH), Mean Corpuscular Hemoglobin Concentration (MCHC) values were decreased because of the cobalt chloride exposure. Statistical analyses showed that the differences in RBC, MCV and MCH were very important (p<0.01) in ESR; important (p>0.05) but the others were not.