Plants light- to yellowish-green, glossy, in dense mats. Stems to 20 × 1--3(4) mm, prostrate, complanate-foliate, rarely somewhat julaceous. Leaves imbricate, rarely distant, erect or spreading, often slightly undulate, sometimes secund with apices pointing toward substratum, usually asymmetric, 0.7--2.6 × 0.3--1.2 mm, oblong-ovate or ovate-lanceolate, slenderly acuminate, margins plane or often narrowly recurved nearly to apex, usually entire or with a few serrulations at apices; costa short and double, ending a short distance above leaf base, rarely with one branch reaching leaf middle, or costa sometimes lacking; leaf cells smooth, walls of basal cells pitted; median cells 96--168 × 4--10 μm; decurrent alar region triangular in outline, consisting of 1--5 vertical rows of rectangular cells, 40--100 × 9--29 μm, terminating at the base in a single cell. Specialized asexual reproduction by propagula usually present, 40--86 × 8--14 μm, consisting of 3--6 cells. Sexual condition autoicous, often fruiting. Seta orange-brown to red, straight or curved, 1--1.6 cm. Capsule light- to orange-brown when mature, erect to cernuous, straight to arcuate, 0.5--2.0 × 0.4--0.7 mm, when dry smooth or rarely wrinkled when arcuate; operculum conic to short-rostrate, 0.3--0.8 mm; cilia 1--3, sometimes lacking. Spores 9--14 μm.

Capsules mature summer. Usually in coniferous woods on rotten logs, stumps, bases of trees, humus or soil, frequently overlying boulders and cliffs; 75--3050 m; Greenland; Alta., B.C., Man., N.B., Nfld., N.W.T., Nun., N.S., Ont., P.E.I., Que., Yukon; Alaska, Calif., Colo., Conn., Idaho, Iowa, Maine, Mass., Mich., Minn., Mont., N.H., N.J., N.Mex., N.Y., N.C., Ohio; Oreg., Pa., Tenn., Vt., Va., Wash., W.Va., Wis.; Europe; Asia.

Plagiothecium laetum has a past history (R. R. Ireland 1969) of being confused with P. denticulatum in North America. The small leaves, 0.7--2.6 mm, with narrower median cells, 4--10 μm wide, the decurrencies that are triangular in outline and composed of rectangular cells, and the usually smooth capsules will distinguish P. laetum from P. denticulatum, which has larger leaves, 1.5--4 mm, broader median cells, 12--21 μm wide, decurrencies usually oval in outline and composed of some oval cells, and capsules striate when dry. I prefer to include Plagiothecium curvifolium Schliephacke in the synonymy of P. laetum. Plants referred to P. curvifolium have smooth secund leaves pointing toward the substratum, decurrencies of 3--5 rows of cells, composed of rectangular cells mixed with some inflated, quadrate or spherical cells, and capsules usually cernuous and often arcuate. However, I have found that these characters do not always correlate and the plants seem best considered environmental forms of the variable P. laetum.