The subject of this study was investigation of red blood cell alterations in diploid and triploid rainbow trout (Oncorhynchus mykiss). The triploid rainbow trout were obtained through the application of heat shock of 26.5°C for 20 min to eggs 20 min post fertilization. The diploids (control) were not subject to shock. The 3n and 2n fingerlings of rainbow trout were reared under the same condition (environment and feeding). Fish ploidal level was determined by erythrocyte size measurement and gill nucleolus counting (NORs). Triploidy rate were achieved about 80.86% in heat shocked treatment. It was found that the percentage of erythrocytes with segmented nuclei in triploid fish was 13.52% and was significantly higher (P<0.01) than in diploid fish (1.06%). In addition amitosis of red blood cells was only seen in triploids (0.57%). It seems that increase of nuclear segmentation is one of the important characteristics for triploid rainbow trout.
