Measurements of erythrocytes and their nuclei were made in 8 species of fishes, 5 species of Amphibia, and 33 species of reptiles. The cytosomes of elasmo -branchs are almost twice the size of those in teleosts, being 19 [mu] to 20 [mu] by 12 [mu] to 14 [mu]. The cytosomes of Opsanus and Lophius are almost as wide as long, while in other fishes they are much longer than wide. The cytosome measurements in the two anurans, Hyla and Bufo, are similar to those in elasmobranchs while those in Rana are larger. Cytosomes in the crocodile, turtles and some lizards are also similar in size to those in the elasmobranchs. The cytosomes of snakes are large (15.5 [mu] to 23 [mu]), those of Bothrops being the largest of all reptiles studied. Based on the assumption that the least diameter of the erythrocyte indicated the minimal diameter of the capillaries, the largest capillaries are found in the elasmobranchs and the smallest in the teleosts.