Plants green to red-brown. Stems often more than 3 cm, central strand present. Stem leaves appressed when dry, strongly recurved and keeled when moist, monomorphic, ovate-triangular, adaxially grooved along costa, 2-2.5 mm, base scarcely differentiated in shape to ovate, margins nearly plane to recurved at mid leaf, entire, apex broadly acute; costa percurrent to short-excurrent, not fragile, weakly tapering, widened pad of cells absent, adaxial costal cells elongate, 2-3 cells wide at mid leaf, guide cells in 1 layer; basal laminal cells differentiated medially, rectangular, walls thickened and often porose; distal laminal cells 13-15 µm wide, 1:1, papillae usually present, simple, often large, 1-3 centered over lumens, lumens angular, walls irregularly thickened and often trigonous but trigones smaller than lumens, convex on both sides, 1-stratose. Specialized asexual reproduction absent. Sexual condition sterile in range of flora. Sporophytes unknown. Distal laminal KOH reaction dark red.

Cliffs, outcrops, canyons, limestone, tundra meadow; low to moderate elevations (20-1300 m); Nunavut; Alaska; w Europe.

Didymodon maximus is rare and apparently disjunctive to the western British Isles (W. C. Steere and G. W. Scotter 1978).