Carassius carassius is one of the fresh water fish species living in Turkey under a natural dissemination. The habitat of this species which is in LC category in the UICN list has recently decreased in parallel with destruction of the ecological conditions of the wetland areas. C. gibelio is an alien species for Turkey and the Northern Europe. It is an invader species disseminated in all over the Europe and Turkey by means of its high tolerance ability against the ecological conditions and with the help of a successful reproduction strategy (gynogenetic). One of the natural habitats of C. carassius is Akgol in the city of Adapazari. However, an invader species, C. gibelio has also been adapted to the same environment, wherein these two species with close characteristics live together in Akgol. Blood samples were received from the 4 C. gibelio and 3 C. carassius specimens using the Laparatomy method wherein 4 blood smear preparations were prepared from each individual. Among the preparations dyed using Gimse method, the photographs of almost 100 erythrocyte cells were taken and computerized. Using the images obtained from the computer, the width and length of the erythrocyte and erythrocyte nucleus were measured by the Spot advanced program which enabled the calculation of the erythrocyte areas, erythrocyte volumes and the area and volumes of the erythrocyte nucleus. The both species with equal chromosome numbers, the erythrocyte and erythrocyte nucleus areas and volumes of C. gibelio were found to be larger than C. carassius. Particularly the size difference in terms of erythrocyte nucleus area indicates that C. gibelio can display a higher power of adaptation by means of the selective gene implication against the ecologically hard conditions.