The brain weights of 51 species of Chiroptera belonging to 10 families were compared using the allometry formula and based on the brain weights of some of the most primitive recent Insectivores. Estimates for the degree of encephalization of the various systematic and dietary groups of bats were given in the form of progression indices. A certain amount of between-group overlap was observed.The lowest encephalized group is formed by the Emballonuridae, Molossidae, Vespertilionidae, Rhinolophidae, and Hipposideridae; a median position is occupied by the Nycteridae and Phyllostomatidae; and the highest position by the Desmodontidae and Pteropidae. The Noctilionidae are present in the lowest as well as in the highest groups in relation to differences in feeding habits of the various species. The following ascending order of encephalization was found: insect-eating forms, nectar feeders, frugivorous Microchiroptera, piscivorous species, bloodsuckers, and frugivorous Megachiroptera.The encephalization indices for the Chiroptera do not exceed the upper limit attained by progressive Insectivores while they reach to some extent into the lowest zone of the Prosimians.Before one can ascertain the true nature of the relationship between systematic and dietary groups and encephalization, a detailed investigation of the brain structures in various species must be carried out; this is now being done.