Perennials; (caudex simple or branched, covered with persistent petiolar remains); glabrous or sparsely to densely pubescent, trichomes simple, sometimes subsetiform, (0.3-2 mm), these rarely mixed with fewer, short-stalked, forked ones. Stems simple or few to numerous from base (caudex), erect or ascending, unbranched, 0.5-2.5(-3.6) dm, (glabrous throughout or pubescent along proximal 1/2, trichomes simple). Basal leaves: petiole 0.3-2(-3.5) cm, (glabrous or ciliate); blade narrowly oblanceolate, spatulate, to obovate, 0.4-2(-3) cm × 3-10(-15) mm, margins entire, (ciliate or not), apex obtuse, surfaces glabrous or sparsely to densely pubescent, trichomes simple, rarely mixed with forked ones. Cauline leaves (1 or) 2-5(-7); blade linear, oblanceolate, oblong, or obovate, 0.5-2.2(-3) cm × 2-8 (-12) mm, base not auriculate, margins entire, (sometimes ciliate), apex obtuse, surfaces glabrous or, rarely, sparsely pubescent. Racemes simple, (dense or lax). Fruiting pedicels ascending to divaricate, (2-)5-20(-30) mm, (glabrous). Flowers: sepals (greenish) oblong, 2.5-3.5 × 1.2-1.5 mm, lateral pair saccate basally, (glabrous); petals white, oblanceolate to spatulate, (5-)6-8 × 2-4 mm, apex rounded; filaments 2.5-4 mm; anthers oblong, 0.6-1 mm. Fruits ascending to suberect, (not appressed to rachis), slightly torulose, sometimes slightly curved, (0.7-)1-2.5(-2.8) cm × 0.8-1.2(-1.5) mm; valves each with distinct midvein extending full length; ovules 10-24 per ovary; style 0.5-1.2(-2) mm, (slender). Seeds not winged, broadly ovate, 0.6-1.2(-1.5) × 0.5-1 mm. 2n = 32. Flowering Apr-Aug. Alpine and subalpine meadows and slopes, open woods, steep slopes and cliffs, mossy mats, dry or moist slopes and hillsides, ridge crests; 500-3200 m; Alta., B.C.; Idaho, Mont., Utah, Wash., Wyo. Arabis nuttallii, the second most widespread native North American species of the genus, is variable in density of indumentum, types of trichomes, plant height, pedicel length, fruit width, leaf shape, flowering and fruiting time, and elevation. As indicated by R. C. Rollins (1941, 1993), the variation seems to be sporadic and does not correlate with clear-cut entities and, therefore, the species cannot be subdivided meaningfully.