Plants: in slender, soft mats. Stems: creeping; central strand cells small, walls thin; axillary hair distal cells elongate. Stem: and branch leaves similar, soft, not complanate, slightly concave; costa 10–30 µm wide at base; alar region reaching from margin 65% distance to costa at insertion; medial laminal cells 12–55 × 7–12 µm, 3–5:1, walls firm, slightly incrassate, eporose. Perichaetia: with inner leaves suddenly narrowed to apex, plicate, apex acuminate. Seta: reddish, elongate, 1–2.5 cm. Capsule: 1.5–2 mm, neck well developed; operculum conic. Spores: spheric, smooth to minutely papillose.

Tree trunks, rotten wood, rock, soil, (sometimes salty) swamps to xeric habitats. low to high elevations (0-3000 m). Greenland, Alta., B.C., Man., N.B., Nfld. and Labr., N.W.T., N.S., Nunavut, Ont., P.E.I., Que., Sask., Yukon, Alaska, Ariz., Ark., Calif., Colo., Conn., Del., Fla., Ga., Idaho, Ill., Ind., Iowa, Kans., Ky., La., Maine, Md., Mass., Mich., Minn., Miss., Mo., Mont., Nebr., Nev., N.H., N.J., N.Mex., N.Y., N.C., N.Dak., Ohio, Okla., Oreg., Pa., R.I., S.C., S.Dak., Tenn., Tex., Utah, Vt., Va., Wash., W.Va., Wis., Wyo., Mexico, Central America, South America, Europe, Asia, n Africa, Pacific Islands (New Zealand), Australia.

Variable features in Amblystegium serpens include the serration of the leaf margin, which ranges from entire to serrulate or even serrate; length of the costa, from very short (less than 1/5 of leaf length) to subpercurrent; the subquadrate to transversely elongate alar cells; and leaf stance, which ranges from erect to spreading. These last two characters have sometimes been used to recognize a separate taxon, var. juratzkanum. However, the continuous range of variation of these characters precludes any infraspecific division within A. serpens, which is a fairly well-circumscribed species with comparatively low levels of plasticity with regard to species of Hygroamblystegium.