Enzyme electrophoresis was employed to ascertain the number of loci encoding plastid phosphoglucose isomerase (PGI) in species representing all sections of North AmericanCoreopsis.Several species from each of the closely related generaBidens, Coreocarpus, Cosmos, andThelespermawere also examined. Species in nine of the 11 sections of North AmericanCoreopsishave two isozymes for plastid PGI, and nearly all species examined in the four other genera also have two (one species has three) isozymes. Since most diploid vascular plants have one plastid PGI isozyme, a gene duplication probably occurred in an ancestor that is common toCoreopsisand the other four genera. That is, two isozymes represent the ancestral number forCoreopsis.The two sections (ElectraandAnathysana) apparently lacking the duplication are closely related woody plants restricted largely to Mexico. One gene encoding plastid PGI ostensibly was silenced in a common ancestor of these two sections. This is concordant with other data suggesting a close relationship between the two sections, i.e., they appear to represent a monophyletic group. The electrophoretic data also indicate that 1) the enigmatic monotypic sectionSilphidiumis more closely related to eastern North American sections and not derived from sectionElectra; and 2) sectionAnathysanais not ancestral to the three California sectionsLeptosyne, Pugiopappus, andTuckermannia; rather, it represents a terminal element closely related to and possibly derived from sectionElectra.