Orthothecium acuminatum is distinguished by stems 0.5 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.