Herbs, annual. Stems erect, 10–200(–240) mm, glandular-puberulent or glandular-pubescent. Leaves basal and cauline, basal in rosette, cauline reduced distally; petiole absent; blade narrowly elliptic, sometimes obovate to oblanceolate, 2–30(–55) × 1–10(–16) mm, margins entire, sometimes crenate to serrate, plane, not ciliate, apex rounded to acute, surfaces: proximals glabrous, distals glandular-puberulent or glandular-pubescent. Pedicels 1–4(–7 on proximal) mm in fruit. Flowers 1 per node, chasmogamous. Calyces inflated in fruit, 5–14 mm, glandular-puberulent to glandular-pubescent or ribs almost tomentose and viscid, lobes subequal, apex rounded and apiculate or acute, intercostal areas white. Corollas yellow with reddish brown spots near mouth, palate ridges yellow, throat floor puberulent, tube-throat (9–)10–23 mm, limb 8–26 mm diam., not bilabiate. Anthers included, glabrous, rarely minutely puberulent. Styles minutely puberulent. Stigmas included, lobes equal. Capsules 6.5–13(–14) mm. 2n = 16.
Phenology: Flowering Apr–Jun.
Habitat: Sandy open or disturbed areas, among shrubs.
Elevation: 70–150 m.
Diplacus vandenbergensis is endemic to Santa Barbara County, mostly on the north side of the city of Lompoc (La Purisima Mission State Historic Park; also on and near Vandenberg Air Force Base) plus one other locality about 10 kilometers farther west (Santa Ynez Valley, 8 miles west of Buellton). Thompson described Mimulus fremontii var. vandenbergensis as a geographically distinct variant of M. fremontii. The discontinuous morphological difference, allopatric/parapatric geographical distribution, and lack of intergrading populations support recognition of the plants with yellow corollas at specific rank. Plants with yellow corollas are found in D. bicolor, D. mephiticus, and D. parryi, but these morphs are found through most of the geographic ranges of these taxa and sometimes are intermixed in populations.