Plants medium-sized, green, yellow-green or brownish to brown-red (clear red rarely seen in extremely exposed habitats); branch and shoot apices not pencil-like; cells of stem epidermis not widened; pseudoparaphyllia ovate-triangular to lanceolate, sometimes irregular; axillary hairs with 1--4-celled distal portion, hyaline when young. Stem leaves falcate or sometimes straight, narrowly ovate to triangular-ovate and gradually narrowed to acuminate apex, concave or slightly so; margins denticulate, sometimes only in part or indistinctly so; costa ending at mid-leaf or to 80% distally in leaf (occasionally shorter or double); alar cells in usually indistinctly delimited transverse, narrowly triangular group which more or less reaches costa, not decurrent, supra-alar cells weakly differentiated. Sexual condition autoicous.

Warnstorfia fluitans is recognized by its somewhat indistinctly delimited transverse, ± narrowly triangular alar groups in the stem leaves, and by being autoicous. The other autoicous species in the genus, W. pseudostraminea, has alar groups that are sometimes transversely triangular (though more broadly so than in W. fluitans), but in some or most leaves, the alar and the large supra-alar cells together form an oval or rectangular group along the lower leaf margins. Warnstorfia pseudostraminea has usually ± straight to slightly falcate stem leaves, often with obtuse leaf apices having short cells, and the leaf apex is commonly hooked. However, neither of the last mentioned features are constant in W. pseudostraminea, and occasional (rare) specimens of W. fluitans have the same character states.

Mineral-poor and acid, but sometimes nutrient-rich habitats, poor fens or bog pools, depressions in rocks, rarely on rocks with trickling water; 0--3440 m; Greenland; Alta., B.C., Man., N.B., Nfld., N.S., N.W.T., Nun., Ont., P.E.I., Que.; Alaska, Colo., Conn., Maine, Mass., Mich., Minn., N.H., N.J., N.Y., N.C., Ohio, Pa., Vt., Va., Wash., Wis.; South America, Eurasia (including Papua New Guinea), s and e Africa, Atlantic Islands, Australia, Pacific Islands (New Zealand), Kerguelen Islands.

A variety of W. fluitans, var. falcata (C. E. O. Jensen) H. A. Crum & L. E. Anderson (Hypnum fluitans var. falcatum Schimper, hom. illeg., Drepanocladus fluitans var. uncatus H. A. Crum, Steere, & L. E. Anderson) has been recognized. However, because no type material seems to be extant, the status of this taxon cannot be evaluated.