Plants to 6 cm. Stem leaves erect-spreading, 3-ranked, flexuose when dry, 0.8–1.5 mm; basal laminal cells oblong to elongate; distal cells quadrate to short-rectangular. Perichaetial leaves somewhat larger than stem leaves. Seta 0.8–2.4 cm. Capsule 0.6–1 mm; exostome teeth lanceolate, incurved, truncate. Spores 33–48 µm.
Phenology: Capsules mature summer.
Habitat: Edges of pools in calcareous rich fens
Elevation: low elevations
Greenland, Alta., B.C., Man., N.B., Nfld. and Labr. (Nfld.), N.W.T., Nunavut, Ont., Que., Sask., Yukon, Alaska, Calif., Ind., Iowa, Mich., Minn., Mont., N.Y., Europe, Asia, Atlantic Islands (Iceland).
Catoscopium nigritum is characterized by its dense turfs of reddish black stems with erect, lanceolate leaves. When fertile, the small, shiny, black capsules with small mouth, reduced peristome, and inclined posture clearly define the species.