Electrophoretic serum protein analysis reveals that the gynogenetic species Poecilia formosa possesses two albumin bands that are identical with those of laboratory-produced hybrids of its two sympatric species, P. latipinna and P. mexicana. The presumed parental species possess single albumins that are different in electrophoretic mobility. Studies involving the mixing of P. formosa, P. latipinna, P. mexicana, and the hybrid P. mexicana x P. latipinna in various combinations demonstrate the electrophoretic identity of the two albumins of P. formosa with those of its presumed parental species. These results substantiate the conclusion, based on morphological and reproductive evidence, that P. formosa arose by hybridization of P. latipinna and P. mexicana.