The article presents the results of the analysis of the genetic structure of the populations of <i>Carassius</i> <i>gibelio</i> (Bloch, 1782) and <i>Carassius</i> <i>carassius</i> (Linnaeus, 1758) by the cytometric method by the size of the area of erythrocyte nuclei. The relevance of the work is due to the discussion on the formation of crucian as one species or several ecological forms and the disappearance of crucian as a species in its natural habitat. We compared crucian carp from different types of water bodies: the Tura river, the Niсa river (the left tributary of the Tura), Lake Krivoe (the Tura oxbow lake, periodically communicating with it), Lake Srednee (isolated, not connected with the Tura for more than 70 years). In all studied water bodies, the diploid form of silver crucian dominates. The proportion of triploids in the Nica river and Lake Krivoe did not exceed 5%, in the lake. The average was equal to 20%, and the maximum (30%) was noted in the Tura. The ratio of cytometrically determined diploids and triploids was: in the river Tura - 2: 1, in the river Nica – 19:1, in Lake Krivoe – 17:1, in Lake Srednee 3:1. At the same time, from 5 to 18% of individuals were not determined by ploidy, their largest number was noted in the sample from Lake Srednee. When comparing the area of erythrocyte nuclei in silver and gold crucian from the lake. The average overlap zone was found at the level of 45–54 µm2, which suggests natural hybridization of the populations of these species.