Phytoneuron 2012-39: 34. 2012.
Annuals, fibrous-rooted. Stems erect to ascending-erect, simple or diffusely branched, 4-angled, 7–20 cm, glabrous. Leaves basal and cauline; petiole 0 mm; blade palmately 3–5 veined, ovate to broadly ovate, 10–20 × 7–11 mm, base rounded to truncate, margins serrate-denticulate, apex acute, surfaces glabrous. Flowers herkogamous, 2–20, from proximal to distal nodes. Fruiting pedicels divergent-arcuate, sometimes becoming deflexed, 10–23 mm, longer than subtending leaves, glabrous. Fruiting calyces campanulate, 7–9 mm, margins subtruncate, glabrous, lobes reduced, subequal. Corollas pale-pink to rose-purple, throat yellow or deep pink externally with 2 yellow spots below abaxial limb, bilaterally symmetric, weakly bilabiate; tube-throat cylindric-funnelform, 9–12 mm, exserted beyond calyx margin; limb expanded 9–12 mm. Styles glabrous. Anthers included, minutely villous-hirsute. Capsules included, (4–) 5–10 mm.
Phenology: Flowering Apr–Jul.
Habitat: Grassy slopes, sandy terraces, marshy places, lakeshores, creek sides, seep edges, shaded canyon slopes, roadcuts and roadsides, woodlands of Pseudotsuga menziesii-Quercus chrysolepis, oak-gray pine, or interior live oak.
Elevation: 200–2000 m.
Mimulus acutidens Greene is heterotypic with the South American M. acutidens Reiche, which is a later homonym.