Lianas, twining, to 5 m, woody. Young stem ribbed, tomentose. Leaves: petiole 1-2.5 cm. Leaf blade ovate to reniform, 4-12 × 3-10 cm, base cordate, sinus depth 2-6 cm, apex obtuse to rounded; surfaces abaxially tomentose; venation palmate-pinnate. Inflorescences on new growth, axillary, solitary flowers; peduncle bracteolate, 1-4 cm; bracteoles reniform, to 8 mm. Flowers: calyx brown-purple, bent; utricle pendent, ellipsoid to narrowly ellipsoid, 0.5-5 × 1-1.5 cm; syrinx absent; tube bent, cylindric, 1-3 × 1-1.7 cm; annulus absent; limb purplish, 3-lobed, lobes 2 × 2 cm, glabrous; gynostemium 3-lobed, cylindric, 5-10 mm; anthers 6; ovary 6-locular, 1-4 cm. Capsule ovoid to cylindric, 5-6 × 1-3 cm, dehiscence basipetal; valves 6; septa entire, not attached to valves. Seeds flat, triangular, 1-1.2 × 1 cm. 2n = 28, 32.
Phenology: Flowering spring.
Habitat: Along streams, in forest thickets, chaparral
Elevation: 50-700(-1000) m
The leaves of Aristolochia californica are eaten by larvae of the western swallowtail butterfly, Battus philenor hirsuta (Skinner) (W. H. Howe 1975).