Andromonoecy, a sexual system in which all individuals have both bisexual and male flowers, is found in at least four North American genera of Cleomaceae: Cleome L., Cleomella DC., Polanisia Raf., and Tarenaya Raf. This can be easily missed because the terms andromonoecious and andromonoecy are largely absent from the literature on these taxa. We conducted a series of experiments on how fruit set, mineral nutrition, mating, and herbivory affect male flower production in a cultivar of the North American cleomid Polanisia dodecandra (L.) DC. subsp. trachysperma (Torr. & A. Gray) Iltis. When we completely prevented fruit set during bisexual flower production, no male flowers were produced. If we allowed fruit maturation, plants would eventually switch to producing male flowers, then switch back to bisexual, and then continue alternating floral sex types as long as we allowed them to grow. Withholding mineral nutrients and simulating leaf herbivory each hastened the initial switch from bisexual to male flowers, but self-pollination did not delay it. Our results strongly indicate that in P. dodecandra subsp. trachysperma, male flower production is an evolutionary response to the regular occurrence of variation during the flowering season in the availability of photosynthates necessary for fruit maturation. It is likely that male flowers do contribute to male reproductive success through increased pollen export, but do not contribute to female reproductive success through increased deposition of pollen on the stigmas of bisexual flowers. More complete knowledge of the sexual systems and infrageneric relationships among North American cleomids, and in Cleomaceae as a whole, should provide interesting new insights into the evolution of sexual systems in angiosperms.