Plants loosely cespitose; rhizomes long. Culms obtusely trigonous in cross section, 30-90 cm, smooth or scabrous-angled distally. Leaves: basal sheaths reddish purple; ligules as wide as long to wider than long; blades dark green, wiry, filiform, with involute margins, 0.5-2.5 mm wide, glabrous. Inflorescences 3-20 cm; proximal bract 3-15(-25) cm, shorter than to exceeding inflorescence; proximal 1-2 spikes pistillate, erect, ovoid to cylindric; terminal 1(-2) spikes staminate. Pistillate scales ovate, 3.2-5.6 × 2.6-3.8 mm, shorter than perigynia, margins entire, apex obtuse to acute, awnless. Perigynia ascending, 7-15-veined, veins extending into beak, ovate, 4-6.7 × 2.5-3.4 mm, leathery, apex tapered; beak 0.3-0.9 mm, obscurely bidentulate, glabrous, teeth straight to somewhat outcurved, to 0.3 mm. Stigmas 3. Achenes brown, trigonous, smooth. Fruiting summer. Bogs, often forming extensive stands in Sphagnum-dominated areas, poor fens, sometimes in acidic, sandy, or peaty soils in open swamps, marshes, lakeshores, riverbanks; 0-1200 m; Man., N.B., Nfld. and Labr., N.S., Ont., Que., Sask., Yukon; Ind., Maine, Mich., Minn., N.H., N.Y., N.C., Ohio, Pa., Vt., Wis. Carex oligosperma hybridizes rarely with C. rostrata.