Triploidy induction at fish is characterized by modification of normal diploid chromosome set (2n) into triploid set (3n). Experiments were carried out on biological material from rainbow trout (Oncorhynchus mikiss) during the natural breeding season. Polyploidy was induced by exposing the eggs to heat shock. Blood smear was used as a technical method, to determine diploid and triploid status. Staining of blood smear was performed by Pappenhein method. The erythrocytes area and perimeter measurements done comparatively on triploid and diploid individuals may represent an indicator to determine the ploidy level of individuals. Erythrocytes area (µm²) for triploid individuals is 1,76 times higher than at diploid individuals, and perimeter (µm²) 1,31 for triploid individuals is times higher than in diploid individuals.