Herbs, annual. Stems erect, (10–)30–200(–300) mm, densely glandular-puberulent. Leaves basal and cauline, gradually reduced distally; petiole absent, bases of larger leaves often with petiole-like extensions; blade obovate or oblanceolate, sometimes elliptic, (4.5–)7–25(–32) × 2–12(–15) mm, margins entire, plane, apex rounded to acute, surfaces densely glandular-puberulent. Pedicels 1–4(–5) mm in fruit. Flowers 2 per node, or 1 or 2 per node on 1 plant, chasmogamous. Calyces symmetrically attached to pedicels, not inflated in fruit, (6–)7–11 mm, glandular-puberulent, lobes unequal, apex acute to acuminate, ribs inconspicuous, intercostal areas reddish. Corollas magenta, darker and more reddish in throat and, often, along narrow radiating lines extending from throat onto midveins of lobes, throat usually with a large dark spot on each side of mouth on lateral walls, palate ridges and throat floor yellow with reddish spots, tube-throat 9–15 mm, limb 10–15 mm diam., not bilabiate. Anthers included, glabrous. Styles densely glandular-puberulent distally. Stigmas exserted or at opening of corolla tube-throat, lobes equal. Capsules 7–12 mm. 2n = 16.
Phenology: Flowering May–Aug.
Habitat: Steep, unstable scree slides, talus slopes, gravel slides, cracks in granite cliffs, ridges, washes, steep sand and gravel slopes, canyon bottoms, gravelly road banks, recent burns, desert scrub, chaparral, juniper, pinyon-juniper, lodgepole pine, yellow pine, Jeffrey pine, and Jeffrey pine-western white pine-fir woodlands.
Elevation: (1000–)1300–2900 m.
Populations of Diplacus johnstonii occur in Los Angeles, Riverside, San Bernardino, Santa Barbara, and Ventura counties. The populations in northwestern Los Angeles, Santa Barbara, and Ventura counties were noted by D. M. Thompson (2005) to be intermediate between D. constrictus and D. johnstonii.