The concentration of sodium and potassium ions in erythrocytes in perch Perca fluviatilis L. was compared to that in ten other freshwater fish species. The cation concentration was determined by flame photometry. The participation of erythrocyte cations in the buffer and respiratory processes that promote the specific resistance of perch to unfavorable factors is substantiated. Among the investigated fish species, the highest sodium concentrations in red blood cells were found in perch and pike, who are the most resistant to water acidification. High sodium concentration in red blood cells indicates a high alkalization capacity of the erythrocytes’ inner environment and is one of the mechanisms for enhanced resistance of perch to water acidification. In perch the potassium concentration in erythrocytes is at the lower limit of the range typical for fish resistant to hypoxia. A high potassium level in red blood cells is regarded as one of the mechanisms for promoting the tolerance of perch to hypoxia. Analysis of data from the literature revealed the same kind of participation of erythrocyte sodium in buffer processes and erythrocyte potassium in respiratory processes in marine fish as in freshwater fish. When determining the level of hematocrit, higher values of this parameter were found in perch as compared to other freshwater fish. Other known physiological and biochemical mechanisms for promoting total and specific resistance of perch to detrimental factors are also analyzed in the paper.