The article presents the results of the study on nuclear damages or injuries of red blood cells (RBCs) and asymmetry of meristic characters of the European perch (Perca fluviatilis Linnaeus 1758) that lives in small and medium rivers of Rivne Region. The research has found that the species representatives living in the small rivers demonstrated worse cytogenetic homeostasis indicators compared to the fish from the medium rivers: the lowest frequency of P. fluviatilis’ nuclear damages appeared to be 4.25 times higher for the individuals living in the small rivers given the noticeable excess of spontaneous mutations by average damage frequency. Fish morphological homeostasis demonstrates a tendency to injuries in small rivers as well. The functional relationships between the studied parameters which have been established by the author were “close” for the fish from the small rivers and primary “moderate” for the fish from the medium rivers. It is concluded that the enhanced activity of the stressors existing in the water environment affects the physiological state of the P. fluviatilis species.