Saurauia veraguensis in Costa Rica is morphologically androdioecious but functionally dioecious. Both staminate and pistillate flowers bear pollen. The pollen in the staminate flowers is tricolporate, and smaller than the inaperturate pollen borne by the pistillate flowers. The inaperturate pollen probably plays a role in pollination by serving as a reward, but it does not germinate on the stigmas and is therefore non-functional in fertilization. Dioecy in Saurauia may have evolved via androdioecy, although other evolutionary pathways such as via gynodioecy cannot be ruled out. Saurauia Willd. (Dilleniaceae) is a large genus of smallto medium-sized trees with approximately 300 species distributed in tropical Asia and America (Willis, 1973). Approximately 65 species occur in the Western Hemisphere from central Mexico to Chile (Hunter, 1966). Taxonomic treatments of the genus imply that the species have perfect flowers (Hunter, 1966). However, Soejarto (1969), in a detailed study of herbarium specimens, found that several species in South America are sexually dimorphic. On the basis of pollen fertility and the size of pistils, he concluded that at least seven species are dioecious and one androdioecious. During the course of our ongoing studies of the reproductive biology of tropical forest plants in Costa Rica, we observed two distinct sexual phenotypes in Saurauia veraguensis, a species not studied by Soejarto. One phenotype appeared to bear only staminate flowers and the other only hermaphroditic flowers, indicating the occurrence of androdioecy. Because androdioecy is presumed to be extremely rare among angiosperms (Charlesworth & Charlesworth, 1978; Bawa, 1980) we investigated the reproductive biology of S. veraguensis. Here we describe the sexual system of S. veraguensis and discuss the evolution of dioecy in the genus. STUDY SITE AND PLANTS Saurauia veraguensis is a small tree, reaching up to 7 m in height. Our field observations were restricted to two populations; one was on the Pacific face of the Cordillera de Tilarafn in the Monteverde cloud forest and the other on the Atlantic slopes of the Volcan Poals, both in northwest Costa Rica at an elevation of 1,500 to 1,600 m. Holdridge et al. (1971) characterized the vegetation of these areas as Lower Montane Moist Forest-Wet Forest Transition. Voucher specimens of the species investigated (Bawa 3001 and 3002) have been deposited in the herbarium of the California Academy of Sciences (CAS), San Francisco. FLOWER MORPHOLOGY AND