Herbs, annual. Stems erect, (20–)30–150(–180) mm, glandular-pubescent and viscid-villous. Leaves usually cauline, relatively even-sized; petiole absent; blade ovate to oblong or narrowly elliptic to narrowly lanceolate, narrowly oblong, or linear, 10–25 × 1–5 mm, margins entire, plane, not ciliate, apex rounded to obtuse or acute, surfaces usually glandular-pubescent and (at least along veins) viscid-villous. Pedicels 1–3 mm in fruit. Flowers 2 per node, or 1 or 2 per node on 1 plant, chasmogamous. Calyces symmetrically attached to pedicels, slightly inflated in fruit, (3–)4–7(–9) mm, glandular-pubescent and viscid-villous (at least along veins), lobes subequal, apex acute to acuminate, ribs dark green to purplish, intercostal areas whitish. Corollas of 2 color forms: (a) dark magenta, purplish, or reddish with palate ridges or whole throat floor yellow, red- or purple-dotted, lateral lobes yellowish inside and (b) yellow with red or purple spots on floor, tube-throat 8–12(–15) mm, tube 1.3–1.9 mm diam. at filament insertion, limb 5–12(–15) mm diam., bilabiate. Anthers exserted, short-hirsute. Styles glabrous or sparsely glandular-puberulent. Stigmas exserted or at opening of corolla tube-throat, lobes subequal to unequal, abaxial to 2 times adaxial. Capsules 5–8 mm.
Phenology: Flowering (May–)Jun–Aug(–Sep).
Habitat: Openings in sagebrush, disturbed slopes, granite outcrops, serpentine substrates, gravelly and sandy soils, sandy moraines, pumice flats, gravelly washes, meadows, shadscale and sagebrush communities, pinyon-juniper and ponderosa pine woodlands.
Elevation: 1300–3700 m.
Diplacus mephiticus occurs in eastern California and west-central Nevada.
Various synonyms treated here are in agreement with D. M. Thompson (2005). Mimulus coccineus (mostly from Eldorado to Tulare counties, California, and, apparently, including Eunanus angustifolius Greene from Mt. Rose, Nevada) includes relatively small, tufted plants at high elevations with relatively small calyces and relatively small, dark red-purple, strongly bilabiate corollas with prominently exserted stamens. Mimulus densus (mostly in Nevada and in Lassen, Nevada, and Plumas counties, California) includes taller plants at lower elevations with a strong tendency to produce populations with all individuals with larger, yellow, nearly regular corollas with more nearly included stamens. Typical Diplacus mephiticus has moderate-sized plants at medium elevations with magenta, bilabiate corollas.
The specific epithet mephiticus alludes to the musky odor of the plants; this has also been noted in plants of Mimulus coccineus and M. densus. Diplacus nanus, in which D. M. Thompson (2005) included D. mephiticus as a variety, apparently does not produce a mephitic odor. Diplacus cusickii also produces a mephitic odor (W. L. Ezell 1971).
The later homonym Mimulus angustifolius (Greene) A. L. Grant 1925, not Hochstetter ex Richard 1850, based on Eunanus angustifolius, pertains here.