Among the Amaranthaceous material of the West Indies, studied for a treatment of this family in the Flora of the Netherlands Antilles, a new species of the genus Alternanthera turned up. The species is easely distinguishable from all other species of this genus in the Caribbean area by its loosely flowered, up to 7.5 cm long spike. The species is described below: Alternanthera arnoldiana Stoff., sp. nov. Herba suberecta basi lignosa. Folia opposita laminis lanceolatis vel lanceolatoovatis usque ad 5 X 1,5 cm, apice acutis, basi acutis vel cuneatis. Inflorescentia terminalis et axillaris spicata usque ad 7,5 cm longa. Tepala 5, subaequalia, lanceolate, apice acuta, 3-3,5 x 1 mm. Stamina 5, epitepalia; pseudosteminodia late ovate staminibus aequilongia vel longiora, margine fimbriate. Ovarium 1 mm longum et 1 mm latum; stylus terminalis. Utriculus circiter 2,5 mm longus et 2 mm latus. Suberect herb, up to 30 cm tall, woody below. Branchlets striate, the younger parts sericeous-strigose, the older ones glabrous or nearly so; internodes up to 5 cm long. Stipules wanting. Leaves simple, opposite, petiolate; petioles 1-2 mm long, canaliculate above, sericeous-strigose; blades ovate-lanceolate to lanceolate, up to 5x1.5 cm, entire, acute at the apex, acute or cuneate at the base, the margin often undulate; the younger blades sericeous-strigose, the older ones glabrous or nearly so, the midrib beneath nearly always provided with scattered hairs. Inflorescence consisting of loosely flowered terminal and axillary spikes; spikes up to 7.5 cm long and 1.5 cm wide, several-flowered; axis more or less striate, sericeous-strigose. Flowers hermaphrodite, sessile, solitary in the axil of a bract and subtended by two bracteoles. Bract persistent, concave, ovate to broadly ovate, long-acuminate to cuspidate at the apex, 1—1.5 X 0.7—1 mm, with same vesture at the axis. Bracteoles slightly unequal, broadly triangular-ovate to suborbicular, cuspidate at the apex, strongly concave, 1.5-2 X 1.4—1.7 mm, membranaceous, shining, glabrous to sparsely strigose at the very base. Tepals 5, subequal, lanceolate, acute at the apex, 3-3.5 X 1 mm, striate, shining, pilose outside. Stamens 5, epitepalous, hypogynous inserted; filaments subulate, basically united into a cup-like tube of about 0.5 mm length; free parts of the filaments about 1 mm long; anthers versatile, bilocular, oblong, about 0.5 mm long at anthesis. Pseudostaminodes broadly ovate, laciniate, equaling or sometimes exceeding the stamens in length. Ovary superior, sessile, subglobose, glabrous, unilocular, about 1 mm long and 1 mm wide; style terminal, more or less conical, about 1 mm long; stigma slightly capitate, papillous. Ovule one, anatropous. Utricle 2.5 mm long and 2 mm wide. Seed subglobose with crustaceous testa. Embryo curved. Endosperm mealy. Type: Curasao, slope of Seroe Christoffel at the side of Knip Plantation, Staffers n. 1157, fl. & fr., Dec. [U] Ecology: in degraded deciduous seasonal forest. Para-types: Seroe Christoffel at the side of Knip Plantation, Arnoldo n. 1014, fl. & fr., Jan. [U]; slope of Seroe Christoffel, 325 m, de Wilde 108a p.p., fl. & fr., Feb. [L] The species is named in honour of Brother M. Arnoldo, student of the flora of the Netherlands Antilles.