Plants scattered or gregarious in usually sparse protonemata, yellow-green to orange-brown. Stems less than 0.2 mm. Leaves erect or spreading, broadly ovate-lanceolate and somewhat clasping at base, acuminate, 0.5-1.1 × 0.14-0.43 mm, margins usually ± incurved beyond the middle, entire proximally and distally entire to serrulate. Sexual condition synoicous. Capsule cleistocarpous or rarely dehiscing by a ring of differentiated cells distal to the equator, orange-brown, globose or slightly flattened, rarely broadly ovoid, with a short, broad apiculus; exothecial cells in 2 layers, stomates confined to distal half, sometimes absent. Spores fewer than 100, various shapes, proximal face only rarely concave, 55-83 × 37-67 µm, orange-brown.
Capsules mature summer-winter, with some capsules persisting through spring with leaves intact or eroded. In sun or partial shade, unfertilized bare soil, sparsely vegetated turf; low to moderate elevations (0-700 m); Ark., Conn., Fla., Ga., La., Maine, Md., Mich., Miss., Mo., N.J., N.C., Pa., Wis.; West Indies (Cuba); e Asia (Japan); Africa (Cameroon).
In immature plants of Micromitrium megalosporum the capsules may be almost completely obscured by the clasping proximal halves of the leaves. Mature specimens can often be detected by a distinctive orange-brown coloration of the capsules, spores, and sometimes of the leaves. The plants frequently persist with intact or dehisced capsules among remnants of leaves, and the dehisced capsules often appear as open cups of orange-brown spores, not obscured by leaves. Chromosome studies have been carried out on only two taxa of Micromitrium (as Nanomitrium): M. tenerum n = 10, 11 (as N. austinii) and M. megalosporum (n = 22). On the basis of this polyploid series and on other characteristics as well, the latter was transferred back to Micromitrium from Ephemerum, species of which have a chromosome number of n = 27 (V. S. Bryan 1957).