Oxygen-transport properties of blood and heart mass of 11 species of bats in the families Phyllostomidae, Mormoopidae, Noctilionidae, Molossidae, and Vespertilionidae were investigated and related to morphological characteristics of the wings and to phylogeny. These 11 species include all West Indian core species, and 85% of the assemblage of bats in the subtropical moist forest of northern Puerto Rico. Hematocrits (48.5%–64%) fall within values expected for bats. Diameter of red blood cells ranged from 4.2µm to 6.2µm. Heart mass (0.32g–0.01g) and hematocrit were significantly correlated to wing shape and wing loading (0.19 g/cm²–0.06 g/cm²), but not to phylogeny. The relationship between hematocrit and wing loading showed a negative slope.