Perennials; (caudex simple or branched, covered with persistent petiolar remains); usually glabrous, rarely sparsely pubescent, trichomes simple, (to 0.5 mm), not bulbous-based. Stems simple or few from base (caudex), erect, unbranched, (0.6-)1.5-3(-4) dm, (glabrous). Basal leaves: petiole 0.3-1.5 cm; blade oblanceolate to obovate, (0.5-)1-3(-4) cm × (2-)3-6 (-10) mm, margins entire, repand, or obtusely dentate, apex obtuse, surfaces sometimes with individual trichomes terminating some or all leaf teeth. Cauline leaves (2 or) 3-5 (or 6); blade oblong, 0.3-1(-1.2) cm × 1-3 mm, base not auriculate, margins entire or repand, apex obtuse, surfaces glabrous. Racemes simple, (dense). Fruiting pedicels ascending to suberect, 3-10(-13) mm. Flowers: sepals (purple), oblong, 4-8 × 1.5-2.5 mm, lateral pair saccate basally; petals purple, spatulate, 8-16 × 2-5 mm, apex obtuse; filaments 4-8 mm; anthers oblong, 1.5-2 mm. Fruits ascending to suberect, not torulose, sometimes slightly curved, 2-4 cm × 1.5-2 mm; valves each with prominent midvein extending full length; ovules 20-34 per ovary; style 0.3-1.5 mm. Seeds narrowly winged distally or, rarely, not winged, oblong, 1.5-2.2 × 1-1.3 mm, wing 0.1-0.2 mm wide. Flowering May-Jun. Serpentine scrap and slopes, red serpentinized soil; of conservation concern; 200-1800 m; Calif., Oreg. Arabis mcdonaldiana is known in California from Del Norte, Mendocino, and Siskiyou counties, and in Oregon from Curry and Jackson counties. It is in the Center for Plant Conservation´s National Collection of Endangered Plants.