Herbs, annual. Stems erect, (10–)20–100(–140) mm, terete. Leaves basal and cauline, relatively even-sized; petiole absent; blade narrowly elliptic to elliptic-lanceolate, (6–)7–27 × 1.2–8(–10) mm, margins entire, plane, apex obtuse to acute or acuminate, surfaces green, often red-purple tinted, usually glabrous, veins and margins glandular-puberulent or ciliate. Pedicels 2–5(–6) mm in fruit. Flowers 2 per node, or 1 or 2 per node on 1 plant, chasmogamous. Calyces asymmetrically attached to pedicel, inflated in fruit, 7–15(–16) mm, glabrous or with glandular-puberulent veins, lobes unequal, apex acuminate, ribs and intercostal areas purplish brown. Corollas salverform-rotate, throat dark purplish brown without internal or external markings, floor purplish brown-pilose, lobes purplish brown basally with red veins, palate ridges absent, tube-throat 9–15(–18) mm, limb 8–11(–14) mm diam., not bilabiate. Anthers included, glabrous or with a few hairs at base of flower pair. Styles sparsely glandular-puberulent. Stigmas included, lobes subequal. Capsules (7–)8–13 mm. 2n = 16.
Phenology: Flowering Apr–May.
Habitat: Gravelly hillsides and slopes, limestone, granite, fine gravel in wash bottoms and edges, commonly with Larrea.
Elevation: 600–900 m.
Diplacus mohavensis is known from San Bernardino County.
Diplacus mohavensis is similar to D. pictus in features of corolla morphology and color patterning, and the pair sometimes has been segregated as Mimulus sect. Mimulastrum A. Gray (for example, by D. M. Thompson 2005). Molecular data (P. M. Beardsley et al. 2004) indicate that D. mohavensis arose from within sect. Eunanus. It is distinct from other species of sect. Eunanus (and similar to D. pictus) in its salverform-rotate corollas with an abrupt tube-throat transition and vein-patterned limb. In D. mohavensis, the limb is purplish brown basally with red, irregularly patterned veins fading into a wide, whitish distal border; in D. pictus, the limb is all white, and the purplish brown vein patterning is more regular and not fading distally.