A quantitative analysis of the brains of 43 bat spp. is presented. Brain components (11) were studied. The species were arranged according to 7 distinct dietary groups; the relative development of the principal components was related to those groups. The importance of neocorticalization as a reflection of evolution of all the bats in contrast to specialization in some species is stressed. This work gives a clearer view of chiropteran progressiveness or primitiveness: the insectivorous forms occupy the least advanced, although most specialized, level; the vampires, the carnivorous species and the flying foxes are at the top of the scale. The importance of behavior and the relative development of the CNS in the hierarchical classification of mammals is stressed.