Plants elliptic to globose, ca. 2 mm. Stem leaves cuspidate, broadly channeled; laminal cells smooth. Seta short, about 0.3 the diameter of the capsule. Spores spheric to elliptic, 30-50 µm, papillose or nearly smooth.

As H. A. Crum (1969) and A. J. Grout (1939) pointed out, American specimens reported as Acaulon rubrum are A. muticum var. rufescens. These plants are often small, rather three-sided and may be confused with A. triquetrum. The leaves of both varieties stain red, at least in blotches, in KOH, and they intergrade to some extent.