The epidermis structure of the studied specimens of Cottus gobio and C. poecilopus from the Morava watershed showed important differences. We noted a lower number of sacciform secretory cells in C. gobio, and also differences in relation to reproductive activity (the decrease in the number of secretory cells during the spawning period in both species). Significant differences were found in the erythrocyte count (1.90 ± 0.15 T.l–1 in C. gobio and 1.57 ± 0.07 T.l–1 in C. poecilopus), whereas the leucocyte count did not differ. A three-day-long exposure in higher temperature and lower oxygen saturated water conditions caused a marked increase in both blood parameters. These differences can be related to the distant physiological and ethological requirements of the species.