Plants solitary or tufted, with bulbils on caudices. Leaves basal; petiole flattened, 1-5 cm; blade ovate to elliptic, 1-4 cm, ± fleshy, base attenuate, margins shallowly serrate to dentate (teeth usually to 1 mm), sparsely ciliate, surfaces tangled, reddish brown-hairy abaxially, glabrate adaxially. Inflorescences 15+-flowered, open thyrses, (flowers ± crowded at tips, becoming more open in fruit), 10-40 cm, purple-tipped stipitate-glandular. Flowers: sepals reflexed, ovate to oblong; petals white, with 2 basal yellow spots (often faded when dried), elliptic to ovate, clawed to nearly not clawed, 1-3.5 mm, ± equaling sepals; filaments club-shaped, not petaloid, (equaling petals); pistils distinct almost to base; ovary superior, (to 1/3 adnate to hypanthium). Capsules green to reddish purple, folliclelike. 2n = 20. Flowering spring-early summer. Open ledges and slopes; 500-2500 m; Idaho, Mont., Oreg., Wash. Micranthes idahoensis appears to hybridize with M. occidentalis where their ranges overlap. Intermediates are abundant in some populations along the Idaho-Montana border and in Montana. This phenomenon may explain the range of filament shapes found in M. occidentalis, from flattened to sightly club-shaped. The issue of the status of M. idahoensis with respect to M. marshallii (D. L. Krause and K. I. Beamish 1972) is best deferred until a thorough study of the whole complex over its entire range is done.