Objective: A study of the effects of triploidy on some haematological indices of rainbow trout. Materials and Methods: Haematological characteristics such as red blood cell (RBC) dimensions, area and volume, RBC and white blood cell counts (WBC), haematocrit (Hct%), hemoglobin (Hb), mean erythrocytic hemoglobin (MEH), mean erythrocytic volume (MEV), mean erythrocytic hemoglobin concentration (MEHC) and RBC abnormalities were measured in 14 and 15 ten month old rainbow trout, respectively. Results: Triploidy significantly increased all morphometric indices containing dimensions, nuclear area and volume of RBCs in compare to diploid fish (p<0.05). The triploid trout had lower numbers with larger RBC sizes. The decrease in RBCs was compensated by an increase in MEV. Thus, triploidy did not affect Hct% (p>0.05). A significant reduction in Hb was observed in triploids (7.4 g/dL) when compared to diploid fish (9.2 g/dL; p<0.05). The MEH values were 82.8 and 116.8 μg/cell in diploids and triploids, respectively (p<0.01). There were no significant differences in MEHC levels between diploids and triploids (p>0.05). Triploids had lower numbers of WBCs and showed higher erythrocytic abnormalities than diploids (p<0.01). Conclusion: Numerous haematological indices of rainbow trout were affected by the ploidy levels, therefore it is expected that optimum culture conditions should vary between diploid and triploid fish.