Historically, the genus Heterotoma has been delimited from the large, cosmopolitan genus Lobelia by a single character, the presence of a nectar spur. Comparative studies between Heterotoma and selected members of Lobelia utilizing macro- and micromorphology of pollen and seed testa, anatomy, and field studies support a hypothesis of three independent derivations of nectar spurs in two sections of Lobelia. The present treatment recognizes 12 species (14 taxa). Het- erotoma lobelioides, including two varieties, is retained as a monotypic genus. The remaining species are transferred to Lobelia sect. Hemipogon. The revision includes keys, descriptions, distribution maps, and illustrations (or references to such) for each species. One new variety (H. lobelioides var. glabra) is described, four new combinations are made (L. aurita, L. endlichii, L. flexuosa var. intermedia, and L. goldmannii), and one new name is proposed (L. volcanica). Heterotoma Zucc. comprises 12 species dis- tributed throughout the pine-oak regions of the Sierra Madre Occidental and Sierra Madre del Sur in western and south-central Mexico south to northern Costa Rica. The genus consists of two morphologically distinct assemblages: 1) H. lobelioides Zucc. (the type species), a large, red-flowered, suffrutescent perennial with a large arcuate nectar spur; and 2) the' 11 re- maining species, which are small, delicate, blue or pink-flowered, herbaceous annuals or peren- nials with gibbose hypanthia or conical-cylin- drical nectar spurs. The genus has undergone revisions by McVaugh (1943) and Wimmer (1953), both as parts of general treatments of the Lobelioideae. These workers delimited Het- erotoma by the irregular hypanthium shape even though this appears to ally taxa which are quite different morphologically. Indeed, both McVaugh and Wimmer noted that this character had created an artificial taxonomic grouping. The present revision was undertaken to pro- vide a systematic study of Heterotoma with spe- cial reference to the reliability of the nectar spur as a generic marker. The study, of necessity, also encompassed much of Lobelia L., especially the North and Central American species that might relate to the two groups mentioned above. To distinguish the members of Heterotoma s.l. from related species of Lobelia, all spurred taxa with available names are discussed as Heterotoma species in the sections preceeding the taxonom- ic treatment. Three new spurred species (Ayers 1987) are, of necessity, discussed as members of Lobelia (L. knoblochii Ayers, L. mcvaughii Ayers, and L. villaregalis Ayers).