Plants in thin mats, yellowish green, glossy. Stems to 20 ´ 0.8--2 mm, complanate-foliate, pectinate, appearing concave because of curvature of upturned leaves, simple or irregularly branched; epidermal layer of stem cells large, thin-walled, enclosing 1-several layers of small, thick-walled cortical cells in the center. Leaves somewhat rigid, crowded and overlapping, erect-spreading and rigidly complanate, smooth, 0.5--1.5 ´ 0.2--0.4 mm, ovate- to oblong-lanceolate, symmetric, acuminate, often abruptly so, margins plane, entire or minutely serrulate; median cells 55--94 ´ 3--6 µm; alar cells not differentiated or sometimes with 1--3 short-rectangular cells only on margins, 8--24 ´ 4--7 µm. Specialized asexual reproduction sometimes present as propagules, 66--96 ´ 6--12 µm, composed of 2--6 cells, green to yellowish green. Sexual condition dioicous. Seta brown to reddish brown, 0.6--1.2 cm. Capsule yellowish brown to orange-brown, erect or rarely ± cernuous, straight, 0.5--1.5 ´ 0.2--0.4 mm, ellipsoid to ovoid, contracted below mouth when dry; operculum conic to obliquely rostrate, 0.4--0.8 mm. Spores 8--12 µm.

Capsules mature fall (Sept.). Acidic cliffs, rock outcrops and boulders in woods; 360--1160 m; B.C., N.B., Nfld, N.W.T., N.S., Ont., Que., Yukon; Alaska, Ark, Conn., Ill., Ky., Maine, Md., Mich., Minn., Mo., N.H., N.J., N.Y., N.C., Ohio, Pa., S.C., Tenn., Utah, Vt., Va., Wis.; Mexico (Distrito Federal); Europe; Asia.

Isopterygiopsis muelleriana is easily distinguished from the other two species in the genus by stems that are complanate-foliate, pectinate, appearing concave on the adaxial surface because of the upturned leaves, and by its epidermal stem cells that are very large and thin-walled. The Utah record is from S. Flowers (1973).