Plants tall, very robust. Rhizomes erect, brownish, thick, somewhat compressed-thickened, praemorse, not brittle. Scapes 1-2, round in cross section, 2.5-6 dm. Bracts held well above ground, spreading horizontally, subsessile; blade very sparsely mottled with dark greenish brown or rarely all green, mottling becoming obscure with age, broadly ovate, 10-22 × 8.7-15 cm, not glossy, often narrowed to falsely petiolate, very short, and narrowly cuneate base 10-20 mm, apex obtuse. Flower erect, odor spicy-musty, musty, or fetid; sepals conspicuous, spreading, often resting on bracts, maroon to green, linear to oblong-lanceolate, 35-47 × 8-10 mm, margins flat, entire, apex acute; petals long-lasting, erect, ± connivent, ± concealing stamens and ovary and partially obscuring stamens, dark purple to red-purple, not spirally twisted, veins obscure, linear, 5-10 × 0.7-1.4 cm, 8-10 times longer than wide, glossy, thick-textured, base linear, margins entire, at first flat but inrolling with age, apex variously acute-obtuse; stamens erect, 12-22 mm; filaments dark maroon, 2-4 mm, slender, widest at base; anthers erect, straight, purple, 12-18 mm, dehiscence introrse; connectives purple, slightly extended 1-1.5 mm beyond anther sacs; ovary dark, ovoid-ellipsoid, 6-angled toward apex, 7.5-12 mm; stigmas erect, divergent-recurved, distinct, purple, sessile, awl-shaped, thickly subulate, 5 mm, thick, fleshy. Fruits dark purple, fragrance unknown, subglobose, 6-angled, almost winged, fleshy. 2n = 10.

Flowering spring--early summer (Mar--Jun). Big-tree (Sequoiadendron) groves and other mixed coniferous-deciduous flatwoods, slightly damper depressions under maples and deciduous shrubs; coastal mountains, oak (Quercus) groves in ravines and otherwise quite arid, almost treeless chaparral, wooded canyon slopes, dense woods near streams; 30--200 m; Calif.

Trillium angustipetalum occurs in the Sierra Nevada from Fresno County north to Placer County (J. D. Freeman 1975). It is disjunct in the coastal mountains and hills of Santa Barbara and San Luis Obispo counties.

B. D. Ness (1993) listed Trillium kurabayashii as a synonym of T. angustipetalum. In bract orientation, color, and texture, and in petal shape, the two are quite different and certainly not the same species. Cytologist Masataka Kurabayashi found chromosomal differences between the two species (reported by J. D. Freeman 1975).