In 7 species of freshwater fishes, with different levels of activity, some with aerial respiration, in addition to aquatic breathing, hematocrit, hemoglobin concentration and erythrocyte number were measured. These parameters were used to calculate mean corpuscular hemoglobin (by weight and percentage) and the mean corpuscular volume. The activity level was correlated positively with the number of erythrocytes and the hematocrit and negarively with the mean corpuscular volume of the red blood cells. Fishes with aquatic respiration exhibit a higher number of red blood cells and hematocrit than fishes with aerial respiration; these showed a higher volume of erythrocytes and hemoglobin content (in weight) in each red blood cell than fishes with aquatic breathing only.