Plants very small, 3--8 mm. Stem leaves 2--4 mm, loosely erect, subulate from an ovate-lanceolate to narrowly obovate base, entire, serrulate along the subula; costa firm, subula flexuose; cells 1-stratose, linear distally, laxly oblong-rhomboidal proximally; perichaetial leaves in transverse section with 6--7 large guide cells closer to the adaxial side, two stereid bands on either side of guide cells in apical sections, but only one (quite disconnected) stereid band at leaf middle and base. Sexual condition paroicous (and reportedly autoicous). Seta stout, pale, 0.6--1 mm, erect. Capsule immersed, ovoid, broadest at the base, whitish, not glossy, spore sac orange, 1--1.5 × 0.6--0.8 mm, strongly apiculate, stomatose in proximal half. Calyptra covering the apiculus, mitrate, shallowly 4-lobed to cucullate, deeply split on one side. Spores 23--30 µm, densely and finely papillose, orange-yellow.

Mature early summer (May--June). Wet soil, sandy swamps; Del., La., Md., Mass., N.J., N.Y., Pa., Tenn.; Europe.

In transverse sections of the leaf, the two largest guide cells are at the center of the costa. Cleistocarpidium palustre can be recognized easily by the extremely long subula, which is flexuose to some degree, the excurrent costa, and the characteristic capsule, small rostrum, and mitrate calyptra. Although an annulus is lacking in the capsule of C. palustre, a non-functional rudimentary ring is present in the rostrum beyond the spore sac region in C. japonicum, which distinguishes these two taxa (H. Deguchi et al. 1994). The latter species was initially recognized as C. palustre (S. Risse 1991) because of the similar gametophytes.