Plants very small, in tufts, brownish proximally, golden green distally. Stems 3-4 cm, 0.5 mm wide, erect to ascending, sparsely branched. Leaves closely appressed-imbricate, straight, broadly ovate, not or slightly plicate, 0.6-0.8 mm; margins plane, slightly serrate distally; apex abruptly very short-acuminate to apiculate; ecostate; basal laminal cells shorter than medial cells, deeply pigmented; medial cells oblong to oblong-rhombic, 35-40 × 9 µm. Specialized asexual reproduction absent. Sporophytes unknown.

Wet calcareous habitats, Arctic tundra; low to moderate elevations; Greenland; N.W.T., Nunavut; Alaska.

Orthothecium acuminatum is distinguished by stems0.5 mm wide and up to 4 cm long, with straight, broadly ovate, abruptly short-acuminate to apiculate leaves. The branches are often slender and stoloniferous, bearing minute leaves. The species is known in mainland North America only from a few localities in the Northwest Territories and one locality in Alaska. W. A. Weber (pers. comm. to R. R. Ireland) suggested that O. diminutivum (Grout) H. A. Crum & L. E. Anderson, described from Colorado, is synonymous with Isopterygiopsis pulchella (Hedwig) Z. Iwatsuki.