Serum proteins of 18 species of neotropical Chiroptera were separated by cellulose poly acetate electrophoresis. Except for differences between non-parous, adult, and gravid females in relative quantities of fractions in the alpha globulin region, resulting electropherograms were independent of sex and age. Albumin and gamma globulin fractions were monomorphic, but polymorphism in fractions of the alpha and beta globulin regions occurred in six species of Colombian and Venezuelan bats and in one species of Puerto Rican bat. Of the species studied, the insular Artibeus jamaicensis presented the most intraspecific variation. Species differed in numbers and mobilities of protein fractions. The electropherogram of Pteronotus parnellii (Mormoopidae) was not unique, whereas Phyllostomus hastatus and P. discolor (Phyllostomatinae) were not only distinct from other Phyllostomatidae but differed significantly from each other. Electropherograms of bats of the families Vespertilionidae and Molossidae were similar to each other and also, among the Phyllostomatidae, to two species of Glossophaginae, three species of Stenoderminae, and one species of Phyllonycterinae. Electrophoretic properties of serum proteins are taxonomically important but may have limited value as indicators of phylogenetic relationships. [Chiroptera; serum proteins; electrophoresis; systematics.]