Rainbow trout were exposed to sublethal concentrations of the pesticide cypermethrin (1/8, 1/4 and 1/2 of the LC50, 0.0082 mg/l) for 21 days at 24-hour intervals. At the end of the exposure period, hematological parameters were investigated. As the concentration of cypermethrin increased, there were increases in red blood cells, hemoglobin concentration, mean corpuscu- lar hemoglobin, mean corpuscular hemoglobin concentration, thrombocyte count and erythro- cyte sedimentation rate. As the exposure increased, there were decreases in packed cell vol- ume, mean corpuscular volume and white blood cell count. Differences among treatments were significant (p<0.05) only for red blood cells, mean corpuscular volume, thrombocyte count and erythrocyte sedimentation rate when exposure rose from 1/8 to 1/2 of the LC50 concentration
