Plants pubescent. Stems (0.3-) 0.7-2.5(-3.5) dm. Basal leaves subsessile or petiole to 0.3 cm; blade oblanceolate, obovate, or oblong, (0.3-)0.7-2(-2.2) cm × (2-)4-7 mm, margins entire or with a tooth on each side, apex obtuse, surfaces uniformly pubescent, trichomes cruciform with fewer, 3-rayed ones. Cauline leaves 2-5; blade similar to but smaller than basal, (base cuneate, not auriculate). Fruiting pedicels stout, 1-2 (-3) mm. Flowers: sepals 0.6-1 × 0.4-0.6 mm; petals spatulate, 1.3-1.7 × 0.5-0.8 mm; filaments 0.5-0.9 mm; anthers 0.1-0.2 mm. Fruits elliptic, obovate, or orbicular, flattened or slightly twisted; valves thickened, veins often prominent, glabrous or uniformly pubescent, trichomes subsessile or short-stalked, cruciform and 3-rayed, sometimes mixed with stouter, simple or forked, subsetiform ones to 0.6 mm, not hooked; septum complete, (membranous); ovules 6-12 per ovary; style 0.1-0.2 mm. Seeds often 2 or more, 0.9-1.2 × 0.6-0.8 mm.

Flowering Feb-May. Open grassy slopes and flats, friable clay, flood plains, adobe and heavy gumbo clay, gypsum clay slopes; 100-900 m; Calif., Oreg.; Mexico (Baja California).

It appears that Athysanus unilateralis is known in Oregon only from Jackson County; it is more widespread in California.