Annuals; (somewhat glaucous), hirsute. Stems unbranched or branched, (often bristly proximally), 0.2-0.9 dm. Basal leaves (soon withered); not rosulate; shortly petiolate; blade oblong-orbicular to obovate, 0.7-1.5 cm, margins dentate. Cauline leaves: blade broadly ovate to obovate, 0.8-1.7 cm × 4-13 mm (smaller distally), base amplexicaul, margins dentate. Racemes ebracteate, (with a terminal cluster of sterile flowers). Fruiting pedicels divaricate, 2-3 mm. Flowers: calyx narrowly campanulate; sepals green to purplish or (sterile flowers) lilac-purple, (narrowly ovate, sterile flowers linear-lanceolate), 3-5 mm, (8-13 mm in sterile flowers), keeled basally (not keeled in sterile flowers; sparsely hirsute in fertile flowers, glabrous in sterile flowers); petals purple (with darker purple veins), 8-11 mm, blade 4-6 × 2.5-3.5 mm, margins not crisped, (flaring), claw 4-5 mm, narrower than blade; stamens in 3 unequal pairs, (purple); filaments: abaxial pair (connate), 4-5 mm, lateral pair 2-3 mm, adaxial pair (connate nearly to apex), 5-6.5 mm; anthers: abaxial and lateral pairs fertile, 1.4-1.8 mm, adaxial pair sterile, 0.4-0.8 mm; gynophore 0.1-0.3 mm. Fruits divaricate, smooth, curved upward, slightly flattened, 1.3-2.5 cm × 2.5-3.5 mm; valves each with prominent midvein, (hispid, trichomes setiform, 0.5-0.8 mm); replum straight; ovules 46-60 per ovary; style 0.2-0.5 mm; stigma slightly 2-lobed. Seeds ovoid (plump), 1.2-1.5 × 0.6-0.8 mm; wing absent. 2n = 28.

Flowering Apr-May. Gravelly sedimentary scree and lag-barrens in chaparral-oak woodlands; of conservation concern; 500-900 m; Calif.

Streptanthus callistus is known from the Mount Hamilton Range of Santa Clara County and is considered to be the most endangered species in the genus.