The genus Chamaesyce (Euphorb iaceae) is represented on all continents, and, while many species are widespread in temperate areas, the genus reaches Its greatest diversity in tropical and subtropical regions. In 1941 Wheeler revised the group in North America exclusive of Mexico and southern Florida, treating it as a subgenus of the Linnaean genus Euphorb ia . He found areas with high concentrations of species centered in Arizona and in Texas, and suggested that a similar concentration might exist in south Florida involving species with a Caribbean affinity. The most recent treatment of the genus in Florida by Small (1933) indicates the same thing, since eighteen species confined to Florida are included in the twenty-nine that he lists for the state. Many of these are from the pinelands around Miami or on the lower Keys, areas with very distinctive plant associations, but not generally rich in endemic species. The plants of south Florida are known to have strong floristic ties both with Cuba and the Yucatan peninsula, and with the Bahama Islands, and it was clear that the Florida species could not be dealt with effectively without considering those of the surrounding area. The Caribbean as a whole - here considered to include southern Florida as well as the Bahamas and the Greater and Lesser Antilles, but excluding Trinidad and the islands of the Dutch West Indies off the coast of Venezuela - does, however, make up a contained unit f lor i st i ca 1 1 y, and is a convenient area for study. It has the added advantage that no synthesis of the genus for this area has been attempted since Boissier's treatment of Euphorb ia sensu lato on a worldwide basis in 1862, although certain regions have been extensively collected since that time, and the genus revised in these localities. Regional floras with relevant material include The Bahama Flora by Britton and Millspaugh (1920), which draws on Millspaugh's strong interest in Chamaesyce, Fawcett and Rendle's Flora of Jamaica (1920), Scientific Survey of Porto Rico and the Virgin Islands by Britton and Wilson (1924), and Flora de Cuba by Alain (1953). There is also material relating to Hispaniola and to Cuba in Urban's Symbolae Antillanae (I898- 1928), and in the reports of Ekman's collecting trips in Haiti (Urban and Ekman, 1929). The consequence of this publication of works covering only small geographic areas has been the description of the same plant under different names from different parts of its range. It is fortunate that much of the type material for these names is still accessible, and it has proved possible in the course of the study to reduce many of them to synonymy. Material from twenty-two herbaria has been examined, and the large number of specimens available of many species has given valuable indications of the extent of morphological variation to be expected and taken into account when considering the validity of names about which there is doubt. Some 4500 sheets have been studied and additional material, as well as valuable type specimens, examined during visits to other institutions. The herbaria from which material has been seen are listed below, together with the abbreviations.