Plants in thin to dense mats, whitish- to yellowish-green. Stems to 50 mm × 0.5--1.5(--3) mm, prostrate. Leaves close, complanate, erect-spreading, often secund at tips, smooth, 0.7--1.8 × 0.2--0.6 mm, ovate to lanceolate, often asymmetric, acuminate; margins plane, serrate to serrulate above leaf middle, serrulate to entire below, rarely entire throughout; median cells 52--151 × 5--8 µm; alar cells short-rectangular to quadrate or transversely elongate, in small groups, 12--38 × 10--20 µm. Specialized asexual reproduction sometimes present as filaments on stems, multicellular, simple or branched, often more than 0.5 mm, green or brown. Sexual condition autoicous. Seta yellow to reddish brown, 0.5--1.5 cm. Capsule light brown to orange-brown, cernuous, rarely erect, arcuate of sometimes straight, 0.5--2 × 0.2--0.5 mm, ovoid to ellipsoid, usually strongly contracted below the mouth when dry; operculum conic-apiculate to obliquely short-rostrate, 0.2--0.4 mm. Spores 9--14 µm.

Capsules mature spring--summer. Dry wooded regions, swamps and wet roadside ditches, rotten logs, stumps, bases of trees, sandy soil; rarely on sedimentary rock; 0--360 m; N.S.; Ala., Ark., Del., D.C., Fla., Ga., Ky., La., Md., Mass., Miss., Mo., N.J., N.Y., N.C., S.C., Tenn., Tex., Va.; Mexico; West Indies; Central America; South America; Europe (Italy).

Isopterygium tenerum is common in Florida and the Gulf Coast, becoming infrequent northward, occurring in scattered localities to southern New York and disjunct to southern Nova Scotia. The species is extremely variable and several varieties have been described from North American plants. These varieties, based on leaf shape and length, are believed to be environmental forms that are not worth formal recognition and are therefore included in the synonomy of I. tenerum. P. L. Redfearn (1956), who did a biometric analysis on the stem leaf variation, reached a similar conclusion. Isopterygium tenerum in its typical form is best distinguished by the usually complanate, small plants with stems 10--20 × 0.5--1.5 mm, filamentous pseudoparaphyllia, leaves ovate-lanceolate, asymmetric, acuminate, close, erect-spreading, 0.7--1.8 mm, alar cells in small groups of short-rectangular to quadrate or transversely elongate cells, asexual reproductive bodies sometimes present on stems, filamentous, multicellular, the cells papillose, setae 0.5--1.5 cm, and capsules ovoid to ellipsoid, inclined to horizontal, usually strongly contracted below the mouth when dry, 0.5--2.0 mm.