The species of Cnidoscolus and Jatropha (Euphorbiaceae) have generally been closely associated in systematic literature since Linnaeus (1753) included representatives of both in his comprehensive genus Jatropha. Adanson (1763) and Mueller (1866) followed the Linnaean concept, whereas Grisebach (1864), Pax and Hoffmann (1931) and MeVaugh (1944, 1945) have upheld Cnidoscolus as generically distinct. MeVaugh presented an impressive array of generic distinctions for Cnidoscolus, including its single (rather than double) floral envelope, repeatedly dichotomous styles, stinging hairs, petiolar glands, and chanmbered pith. The problem of generic definition in the Jatropha-complex, however, is complicated by the fact that a number of groups other than Cnidoscolus have been segregated from ;Jatropha. Britton and Wilson (1924), for instance, placed J. curcas in the genus Curcas and J. gossypifolia in Adenoropium. Small (1913) referred J. dioica to the segregate genus Mozinna. FuLrthermore, Leo6n (1941) has taken two remarkable West Indian species out of Cnidoscolts as a separate genus Victorinia. One of the muain areas of interest in the program of evolutionary studies on Euphorbiaceae at Purdue deals with the comparative morphology of the American genera of Crotonoideae related to Croton and Jatropha. A study by the senior author (1961) includes observations which have a bearing on the Cnidoscolus-Jatropha problem in particular. In some respects, such as stomatal type and venation, there appear to be no significant differences between the genera. The evidence which is especially interesting is that yielded by studies of petiolar anatomy, pollen morphology, and cytology. Petiolar anatomy proved to be significant in demonstrating relationships in Cnidoscolus and Jatropha at both the generic and intrageneric levels. Dehay (1935), in his survey of Euphorbiaceous petiolar anatomy, reported only on Jatropha curcas and did not mention Cnidoscolus, nor do any other workers appear to have done so. Our observations are based on the examination of freehand sections of the petioles of 9 taxa of Jatropha and 2 species of Cnidoscolus. Sections were made at three -levels (base, middle, and junction with blade), stained with phloroglucinol and hydrochloric acid, and mounted in Hoyer's medium (Beeks 1955).