Bulbs 1-3, not clustered on stout, primary rhizomes, ovoid to ± globose, 1-1.6 × 0.9-1.6 cm; outer coats enclosing 1 or more bulbs, reddish brown, membranous, lacking cellular reticulation or cells arranged in 2-3 rows proximal to roots, ± quadrate, without fibers; inner coats pale brown to white, cells obscure, quadrate. Leaves persistent, withering from apex by anthesis, 1, basally sheathing, sheath not extending much above soil surface; blade solid, terete, 10-30(-40) cm × 1-3 mm. Scape persistent, solitary, erect, solid, terete, 7-20(-30) cm × 1-3 mm. Umbel persistent, erect, compact to ± loose, 10-50-flowered, hemispheric, bulbils unknown; spathe bracts persistent, 2-3, 8-10-veined, lanceolate, ± equal, apex acuminate to long-acuminate. Flowers campanulate, 6-10 mm; tepals erect, white or tinged pink with dark red midveins, lanceolate to ovate, ± equal, becoming papery in fruit, margins entire, apex obtuse to acute or apiculate, not conspicuously recurved at tip; stamens included; anthers yellow; pollen yellow; ovary crested; processes 6, prominent, ± triangular, margins erose to ± laciniate; style linear, equaling stamens; stigma capitate, 3-lobed, lobes slender, recurved; pedicel 7-20 mm. Seed coat dull; cells minutely roughened. 2n = 14. Flowering mid Apr--Jun. Serpentine clay soils; 500--1500 m; Calif. Allium diabolense is known only from the southern Coast Ranges and western Transverse Ranges.