Abstract 1. 1. Lactate dehydrogenase (LDH) isozymes of Artibeus jamaicensis, Erophylla bombifrons and Molossus fortis were analyzed by cellulose polyacetate electrophoresis. 2. 2. Heart and liver these bats have similar LDH isozyme profiles. LDH skeletal muscle patterns of A. jamaicensis and E. bombifrons share common features and resemble those of terrestrial mammals and birds which fly only occasionally and in short bursts. Skeletal muscle pattern of M. fortis differ considerably from Arbieus and Erophylla and the similar to those of vespertilionid bats and birds with sustained flight. The cathodal (M) forms of LDH prevalent in rodent and human liver also appeared in bat liver. 3. 3. Liver of fetal and adult E. bombifrons have identical LDH isozyme profiles, whereas fetal heart has a predominance of cathodal (M) forms as opposed to anodal (H) forms in the adult. 4. 4. The isozyme LDH x, unique to the testes and spermatozoa of mature mammals and birds, is present in the adult absent in the testes of juvenile male Artibeus jamaicensis. 5. 5. Skeletal muscle patterns of insectivorous bats of the families Vesperitlionidae and Molossidae, specialized for fast flight and great maneuverability, are similar but differ strikingly from the slower flying, predominantly necto frugivorous bats of the family Phyllostomatidae.