Cap 0.5-3.0 cm broad, convex when young with an incurved margin, at maturity broadly convex, the margin then decurved; surface glabrous, subviscid when fresh, dingy reddish-brown to vinaceous-brown, hygrophanous, fading when dry to buff-brown; context thin, 1-2 mm thick, colored like the cap, unchanging when cut; odor not distinctive; taste slightly bitter.
Gills narrowly attached, sometimes appearing free, close, relatively broad, cream-buff to pale vinaceous-buff; lamellulae up to two-seried.
Stipe 4-10 cm long, 2-3 mm thick, straight, equal, round, hollow, cartilaginous; surface of apex pale vinaceous-brown, glabrous or with faint appressed fibrils, dull reddish-brown below, the base covered with pallid hairs; stipe base fused with those of other fruiting bodies; partial veil absent.
Spores 5.0-6.0 x 2-2.5 µm, smooth, narrowly tear-shaped, inamyloid, possibly slightly amyloid; spore print white.
In dense clusters on rotting conifer stumps and logs; fruiting from after the fall rains to mid-winter; uncommon in the inner San Francisco Bay Area, but collected regularly along California's north coast and in the Sierra.
Gymnopus acervatus is recognized by the dense clusters of small reddish-brown mushrooms that it produces on rotting conifer stumps and logs. In age the color fades, but it can still be recognized by the somewhat slippery, convex cap, persistently incurved margin, and two-toned stipe, pale at the apex, dull reddish-brown at the base. Look-alikes include clustered, lignicolous Mycenas such as M. haematopus, M. galericulata and M. maculata. These can be separated by their more conic to campanulate, striate-margined caps.