Astrocasia, a neotropical genus belonging to the Euphorbiaceae, tribe Phyllantheae, occupies an isolated position in the tribe; a new subtribe, subtribe Astrocasiinae, is described to accommodate it. The genus is interpreted as including five species, of which two require new combinations: Astrocasia jacobinensis and A. austinii. A key, descriptions, distribution maps, and citations of representative specimens are provided. Astrocasia tremula is shown to have a remarkable disjunct distribution, with populations recorded from the Greater Antilles, Yucatan peninsula, western Mexico, Panama, and Venezuela. Astrocasia Robinson and Millspaugh, a small genus of five neotropical species, belongs to the subfam. Phyllanthoideae. The genus was de- scribed by Robinson and Millspaugh (in Mills- paugh 1905) on the basis of a plant collected in Yucatan by Seler. The authors noted that their new genus resembled species of Phyllanthus L., but differed in having a well-developed corolla. The Yucatan species had, in fact, been earlier described by Grisebach (1859) as Phyllanthus tremulus, based on a collection from Jamaica. Mueller Argoviensis (1863, 1866, 1873) created a new sect. Ciccopeltandra within Phyllanthus to accommodate his Phyllanthus jacobinernsis from Bahia, Brazil; this Brazilian plant is clearly con- generic with the Jamaican one. Neither Rob- inson and Millspaugh (in Millspaugh 1905) nor Pax and Hoffmann (1922) appear to have been aware of these earlier treatments of the genus. Johnston (1923) and Standley (in Ferris 1927) added two more species from Mexico, giving the impression that Astrocasia is a genus of Mex- ico and the Greater Antilles. Study of specimens now available, however, shows that Astrocasia extends to South America and includes a total of at least five species.