Plants in, blackish green not very hoary tufts. Stems 1--0.5 cm, central strand present. Stem leaves loosely appressed, twisted when dry, erectopatent when moist, ovate-lanceolate, tapering to acute apex, 2--3 × 0.6--0.8 mm, keeled, margin recurved in mid leaf on both sides, awns short, denticulate, in perichaetial leaves stout and decurrent, costa channeled above, projecting at dorsal side, angled to bluntly winged; distal laminal cells 1-stratose with 2-stratose ridges, margins 2-stratose; medial laminal cells quadrate to short-rectangular, sinuose, thick-walled; basal juxtacostal laminal cells elongate, yellowish, nodulose, thick-walled; basal marginal laminal cells short-rectangular, thickened transverse walls. Gemmae in clusters, short-stalked, usually present in distal leaf axils. Sexual condition dioicous. Seta arcuate, 2--3 mm. Capsule occasionally present, exserted, ovoid, shiny, brown, smooth to slightly striate, exothecial cells thin-walled, annulus present, operculum rostrate, peristome teeth purple, fully developed or slightly split distally, papillose. Calyptra mitrate.

Shaded acidic rock, often along lakes; 200--2000 m; Nfld., Ont.; Mich., Minn., N.Y., Oreg., Wash., Wis.; Africa (South Africa); Europe; Asia (Japan).

Grimmia muehlenbeckii is closely related to G. trichophylla, and in the past it was frequently conceived as being a subspecies or variety of that taxon. H. Deguchi (1978) treated it as a distinct species, followed by A. J. E. Smith (1992). H. C. Greven agrees with these treatments. He has seen many specimens that are remarkably uniform and easy to distinguish from G. trichophylla by their small, ovoid, shiny, dark brown capsules with purple, fully developed peristome teeth. In contrast, the capsules in G. trichophylla are obloid, dull, yellowish-brown, and the peristome teeth are orange and cleft. The gametophyte differs from G. trichophylla by its blackish green tufts, ovate-lanceolate leaves with stout denticulate awns, flattened at base and usually decurrent with some teeth, broadly recurved margins, and costa protruding on the dorsal side with angles.