Plants slender and delicate, aquatic or prostrate, with a conspicuous terminal bud; dark greenish to purplish brown in submerged plants to deep salmon-red in prostrate plants, capitulum quite indistinct$ but with distinct terminal bud. Stems pale green to brown; superficial cortex of 1--2 layers of thin-walled enlarged cells. Stem leaves broadly ovate, 1.5--2(2.5) mm; straight; hyaline cells fibrillose and nearly aporose, with single small pores occasionally found in the distal cell ends on the concave surface. Branches lacking or short and slender. Branch fascicles none or 1 spreading branch. Branch leaves when present similar to stem leaves$ but smaller, 0.8--1.2 mm, hyaline cells fibrillose and mostly aporose, 1--6 irregularly round-shaped membrane gaps in some cells near apex on convex surface. Sexual condition dioicous. Capsules usually immersed in perichaetiale leaves, but may be slightly emergent, pseudostomata absent from capsule surface. Spores 29--41 µm, coarsely papillose on both surfaces, indistinct raised sculpture on distal surface; proximal laesura less than 0.5 spore radius.

Weakly minerotrophic, wet rocks, poor fens; Nfld., N.S., Que.; Maine, Mass., N.H., N.J., N.Y. N.C.; Europe.

Sporophytes rare. Sphagnum pylaesii is distributed as a pioneer species on wet rocks associated with S. tenellum, and S. papillosum, or in poor fens with S. pulchrum, S. majus, and S. papillosum. See also discussion of S. cyclophyllum.