Plants 0.5-1(-2.5) cm. Stems red or reddish brown. Leaves green to dark green, slightly contorted or undulate when dry, obovate or occasionally elliptic, 2.5-5 mm; base long-decurrent; margins reddish brown, multi-stratose with stereid band, toothed distally, teeth paired, long, sharp; apex acute, obtuse, or rounded-acute, cuspidate, cusp toothed; costa percurrent or excurrent, distal abaxial surface smooth, sometimes with few small teeth; medial laminal cells ± isodiametric or short-elongate, 25-40 µm, smaller towards margins, not in rows, not collenchymatous, blue postmortal color absent; marginal cells linear, in (3-)4-5 rows. Sexual condition synoicous. Seta single or multiple. Capsule pale yellow, 1.5-4 mm; operculum rostrate; exostome dark reddish brown, often purplish. Spores 16-24 µm.

Capsules mature late spring. Forests, on humus, logs, rock, base of trees; low to moderate elevations; Alta., B.C., Man., N.B., Nfld. and Labr., N.W.T., N.S., Ont., P.E.I., Que., Sask., Yukon; Alaska, Ariz., Calif., Conn., Ga., Idaho, Ill., Ky., Maine, Md., Mass., Mich., Minn., Mont., N.H., N.Y., Oreg., R.I., S.Dak., Vt., Wash., Wis.; Europe; Asia.

Mnium spinulosum is distinguished by usually obovate leaves with broad, thick, and pigmented margins, and dark reddish brown exostomes. The capsule mouths are dark reddish brown; H. A. Crum and L. E. Anderson (1981) referred to this species as the red-mouthed mnium.