Comparative analysis of Middle Dnepr crucian carp’s population structure was made by means of biochemical gene marking, cytometry and biological analysis. As a result the structure was found, which may by expounded like association of the genetically related species and forms. This association consists of diploid C. auratus (which predominates in population), C. carassius (which was found in the some of north-eastern reservoirs only) and triploid C. gibelio. In Ukraine the homogeneous populations of C. gibelio can be found on the north-east of Eastern Ukraine only. Though some of individuals maybe present with diploid C. auratus. Besides were found hybrid form: diploid C. auratus – C. carassius and presumably tetraploid C. auratus – C. gibelio and С. Gibelio – C. carassius. However in the tetraploid’s spectra were absent a number of specific alleles, that marking C. species – one of the parental species of triploid C. gibelio. Therefore presumptive tetraploids have not difference from C. auratus and diploid C. auratus-C. carassius on the gene markers level. Nevertheless they are neatly diagnose by the erythrocytes which have good difference and are bigger then diploid species erythrocytes in 40 % (C. auratus – C. gibelio) and 100 % (С. gibelio – C. carassius).