Mean erythrocyte number per mm3 of blood among nine species of fishes studied ranged from 680,000 for toadfish to 4,120,000 for white perch. Intermediate values were obtained for eel (2,020,000), searobin, flounder, blackfish, puffer, scup, and bluefish (3,830,000) in that order. In general, fishes which lead a comparatively sedentary existence possess fewer erythrocytes and less hemoglobin per unit volume of blood than the more active types. Within each species, a statistically significant correlation exists between erythrocyte number and hemoglobin content.