Although it is yet to be conclusively established, floral features of many legumes of the mimosoid clade suggest the existence of a highly wasteful late-acting form of genetic self-incompatibility. We evaluated the role this may have played in the evolution of aggregated pollen, restricted stigmas, and polycarpelly in these plants by first investigating the reproductive biology of two species from the southeastern United States: Mimosa microphylla and Albizia julibrissin. Insights gained from detailed study of their reproductive biology informed a wider, critical review of reproductive characters in other members of the clade. Descriptions of the flowers and inflorescences of the two southeastern U.S. species fail to define clearly the functional situation, including that both species are andromonoecious. Pollen of M. microphylla is released as tetrads, whereas polyads of 16 pollen grains characterize A. julibrissin. Diameters of the cup-like stigmas are constrained so that typically only one or a few tetrads or one polyad will fit. These features are consistent with a syndrome involving, in its fullest manifestation, low fruit set, restriction of stigmatic surfaces, pollen aggregation, reduction of ovule number, and polycarpelly. We interpret all of these unusual features as adaptations to circumvent a highly wasteful (of pollen and ovules) form of late-acting self-incompatibility in which rejection of self-pollen occurs in the ovary. These adaptations reduce the likelihood of a mixed load of self- and cross-pollen being delivered. Enhanced efficiency of pollen transfer may also contribute to the evolution of aggregated pollen, but this explanation fails to account for the other unusual features of this syndrome.