Carassius gibelio (Bloch) is documented to have been moved from Asia across Europe in the 17th century since which time it has been reported to exist in fresh water across Europe. The first time this species was reported in Turkey was in 1988 and it has since been reported in fresh water occurring in various sites in Turkey. C. gibelio is an intrusive species that has a negative impact upon natural species occurring in the fresh water sites that it invades. It is known to end up achieving dominance in fresh water sites by not only increasing their total biomass through rapid growth but also competing with the indigenous species. The chief reason for the rapid invasion of this species is thought to be its gynogenetic reproductive ability. Various studies have demonstrated that this species makes up colonies with different genetic traits within the same medium of fresh water as a result of gynogenetic reproduction. A large number of colonies that are reproduced gynogenetically are made up of female individuals. Gynogenetic reproduction is determined upon either the number of chromosomes or the size of erythrocytes. The present study was carried out in seven samples made out of 381 individuals of C. gibelio (20 males and 361 females) cultivated from the darn on the Porsuk river between May and December 2009. About 95% of the population was made up of female individuals. Blood samples were taken off 4 female individuals in December and then preparations of blood smear were arranged. Afterwards, the area and volume of the erythrocyte cell of 50 individuals as well as nuclear area and volume of erythrocyte cells were calculated. These analyses showed the range of erythrocytes of the 4 individuals were 51.31, 58.70, 57.44 and 47.44 mu m(2), respectively. Their range of erythrocyte nucleus were 8.50, 12.41, 8.54 and 7.26 mu m(2), respectively. The volumes of erythrocyte cells were 221, 304, 271 and 195.6 mu m(3), respectively while the volumes of erythrocyte nucleus were 14.66, 30.64, 16.24 and 11.72 mu m(3), respectively. A comparison of nuclear area of the second individual which had the largest nucleus area with those of the remaining 3 individuals determined their areas to be 1.46, 1.45 and 1.71. Previous studies have revealed that nuclear area of erythrocytes of the individuals with triploid (3n) have a much bigger nucleus than individuals with diploid (2n). The population structure consisting of 95% females and the size difference between the erythrocyte nucleus areas displays the gynogenetic reproduction potential of C. gibelio population in Porsuk Dam lake.