Subshrubs. Stems erect, 300–1000(–2000) mm, glandular-puberulent and short-villous. Leaves usually cauline, relatively even-sized; petiole absent; blade elliptic to lanceolate, elliptic-lanceolate, or elliptic-oblanceolate, 25–65(–80) × 4–15(–25) mm, margins entire or serrate, revolute, apex acute to obtuse, abaxial surfaces densely hairy, hairs branched, adaxial glabrescent. Pedicels 5–16 mm in fruit. Flowers 2 per node, chasmogamous. Calyces not inflated in fruit, 22–32 mm, glandular-puberulent and short glandular-villous to hirsute-villous, tube slightly dilated distally, lobes unequal, apex acute, ribs green, intercostal areas light green. Corollas red to orange-red, throat whitish at least on floor, palate ridges orange or white with orange crest, tube-throat 34–45 mm, limb (25–)28–40 mm diam., bilabiate, lobes oblong, apex of adaxial 2 each shallowly, asymmetrically incised. Anthers included, glabrous. Styles minutely glandular. Stigmas included, lobes equal. Capsules 18–28 mm.
Phenology: Flowering Mar–Jun.
Elevation: 400–600 m.
H. E. McMinn (1951), M. C. Tulig and G. L Nesom (2012), and Nesom (2013c) treated Diplacus rutilus at specific rank, but P. A. Munz and D. D. Keck (1973) followed the original assessment of A. L. Grant (1924) in treating it as a variety of the light orange- to pale yellow-orange-flowered D. longiflorus, while R. M. Beeks (1962) and D. M. Thompson (2005) regarded this taxon as only a variant of D. longiflorus, without formal rank. It is maintained here as a distinct, red-flowered species localized in Los Angeles County, California. Diplacus rutilus occurs in pockets in a strip from Whittier and Pomona through North Pasadena westward to near the Ventura County line (the Santa Susana area being the type locality), a distance of almost 60 miles.
Very few of these red-flowered collections can be separated in any feature except corolla color from typical Diplacus longiflorus. Red corollas have not been observed in D. longiflorus outside of Los Angeles County, and D. rutilus might be interpreted as reflecting local introgression in this area from D. puniceus, but the distinctive and tightly coherent geographical distribution of these red-flowered plants and their apparent absence elsewhere in the area where D. ×australis occurs suggest that the origin of D. rutilus is different from that of the highly variable D. ×australis.