A locus influencing floral pigment intensity in the morning glory, Ipomoea purpurea, is polymorphic throughout the southeastern United States. Previous work has suggested that the white allele at this locus has a transmission advantage during mating because of the effect of flower color on pollinator behavior. The experiment described here was designed to determine whether other effects of the W locus may contribute an opposing selective advantage to the dark allele. Dark homozygotes were vegetatively smaller and produced fewer flowers, seed capsules and seeds than either light heterozygotes or white homozygotes. In addition, dark homozygotes produced smaller seeds than heterozygotes, and there is some indication that white homozygotes also produced smaller seeds than heterozygotes. Pleiotropic effects on seed number thus do not seem to contribute to selection opposing the mating advantage associated with the white allele. However, pleiotropic effects on seed size might contribute to overdominance that could stabilize the W locus polymorphism.