Plants densely cespitose. Culms 15-38 cm. Leaves: sheath summits white-hyaline, green ribbed, U-shaped, occasionally prolonged to 4 mm distal to collar; distal ligules 1-3(-3.5) mm; blades 2-4 per fertile culm, 4.5-18 cm × 1.5-2.5 mm. Inflorescences nodding, dense, dark or medium brown, 1-1.2 cm × 5.5-10 mm; proximal internode 1.3-3 mm; 2d internode 0.6-1.5 mm; proximal bracts scalelike to bristlelike. Spikes 3-6, usually individually indistinct, ovoid to broadly ovoid, 4.5-6.5 × 2.5-5.5 mm, base and apex acute to rounded; terminal spike without conspicuous staminate base. Pistillate scales dark brown, usually with pale or green midstripe, ovate, 1.5-2.5(-3.5) mm, usually shorter, narrower than perigynia, margin occasionally hyaline gold, to 0.4 mm wide, apex obtuse or acute. Perigynia ascending-spreading to spreading, gold to dark brown, conspicuously 0-7-veined abaxially, conspicuously 0-4-veined adaxially, ovate, plano-convex, 2.6-3.2 × 0.9-1.3 mm, 0.5-0.65 mm thick, margin unwinged, pithlike tissue usually present in proximal perigynia walls; beak medium or dark brown at tip, often with white-hyaline, cylindric to somewhat flat, abaxial suture with conspicuous white margin, distance from beak tip to achene 1.2-1.6 mm, entire. Achenes ovate to obovate, 1.2-1.5 × 0.8-1 mm, 0.4-0.5 mm thick. 2n = 64

Fruiting summer. Marshes, bogs, wet meadows; 2100-3700 m; Alta., B.C.; Calif., Colo., Idaho, Mont., Nev., Oreg., Utah, Wash., Wyo.

Carex illota is unlike all other members of sect. Ovales in having perigynia lacking wings and margins consistently entire from perigynia base to beak tip.