A new species of Oplonia from the province of Jujuy, Argentina, 0. jujuyensis, is de- scribed, illustrated, and compared with its closest relative. It differs markedly from 0. hutchisonii by the corolla, which is weakly zygomorphic, and by the erect limb, with the posterior lip deeply two- lobed. This is the first report of this genus from Argentina. The present study of the Acanthaceae for the flora of the Province of Jujuy, Argentina, has led to the recognition of a new species of Oplonia from that region. The genus belongs to the subfamily Acanthoideae tribe Odontonemeae in accordance with Lindau's (1895) infrafamilial classification of the Acanthaceae, or in the tribe Justicieae subtribe Odontoneminae if one prefers to follow Breme- kamp's (1965) system of subfamilial classification. It is characterized by flowers that are either solitary in the leaf axils or borne in comparatively few- flowered axillary fascicles with minute bracts. This much reduced inflorescence distinguishes Oplonia from Odontonema Nees, the latter of which has flowers borne in terminal racemes or panicles. The corolla is zygomorphic, occasionally only weakly so, with a slender tube and a two-lipped limb, the pos- terior lip is two-lobed and the anterior lip is three- lobed. There are two stamens, which may be ex- serted or included, and two staminodes. Presently, two species are known from northern Peru, eight from the West Indies, and five from Madagascar. This is the first time that the genus is reported from Argentina. Although the genus Oplonia is easily recognized and defined, the delimitation and distribution of the taxa are more complex and somewhat unusual. Most species grow in dry open habitats as part of scrub forest. The new taxon is found in thicket of tran- sitional forest between the Chaco and Yungas (sensu Cabrera, 1976). The evolution of the genus (Stearn, 1971) would appear to be from a mesophytic spine- less South American ancestor, resembling the small- flowered species of Odontonema, by progressive re- duction in leaf size, by the contraction of the inflo- rescence from a narrow panicle or raceme into a few-flowered axillary fascicle or solitary flower, and by the conversion of lateral branches into spines, by which changes they became xerophytic. We be- lieve that Oplonia jujuyensis, with its reduced leaves, an inflorescence of few flowers in axillary fascicles, small, Odontonema-like flowers, and habitat rep- resents an isolated transition between the genera Odontonema and Oplonia.