Abstract: Heat shocks were applied to rainbow trout eggs, starting between 5h and 9h after fertili-zation. The frequency of occurrence of tetraploid was 5% in fish of twelve-month-old. During. the experiment, the higher temperature resulted in high mortality rate of fish eggs.Being different from the diploid rainbow trout which consists of 22 pairs of metacentric and submetacentric chromosomes, 1 pair of subtelocentric chromosomes and 7 pairs of telocentric chromosomes, the karyotype of the tetraploid rainbow trout consists of 22 sets of metacentric and submetacentrie chromosomes, 1 set of subtelocentric chromosomes and 7 sets of telocentric chromosomes. No mosaic polyploids were found amongst the young troutNine erythrocyte indexes (DNA content, and major axis, minor axis, surface area and volume of cell and nucleus) of diploid and tetraploid rainbow trout were analysed and compared in order to provide the basis of identifying tetraploid in rainbow trout by means of erythrocyte inspection. No differences were found between the tetraploid and the diploid in morphology, anatomy, and growth rate.