Cap 6.0-9.0 (11.0) cm broad, convex, becoming plano-convex to plano-depressed; margin incurved, then decurved, wavy, rimose in dry weather; surface dry, grey-brown, appearing streaked from appressed fibrils over a pallid ground color, the disc sometimes darker and tomentose; context firm, white, unchanging, thin at the margin, up to 1.0 cm thick at the disc; odor and taste mild.
Gills adnexed to notched, distant, broad, up to 1.0 cm, white to cream-yellow at maturity, edges often eroded in age; lamellulae in up to four series.
Stipe 7.0-12.0 cm long, 1.0-2.0 cm thick, more or less equal, occasionally enlarged at the base, stuffed to hollow at maturity; surface pruinose at apex, the lower portion streaked with innate pale-brown fibrils, white rhizomorphs at base; partial veil absent..
Spores 7.5-9.0 x 5.0-6.0 µm, ovate to elliptical, thin-walled, smooth, hilar appendage apparent, inamyloid; spore print white.
Solitary, scattered, to occasionally clustered, on or near rotting logs and stumps, both hardwoods and conifers; fruiting spring and fall; uncommon.
Questionable; some adverse reactions have been reported.
A Tricholoma-like mushoom, Megacollybia fallax is recognized by a greyish-brown, streaked to appressed-fibrillose cap, broad well-spaced gills, often with ragged edges, and a tendency to fruit near or on rotting wood. Molecular studies indicate that it is part of species complex centered around Megacollybia platyphylla, a taxa now believed to be restricted to Europe and Asia. Several similar species occur in the U.S., their geographic distribution an important distinguishing character. Megacollybia fallax occurs in the western United States, while other species occur in the eastern and central U.S. Megacollybia fallax should be compared with species of Pluteus, Tricholoma, as well as larger Mycenas. Pluteus species such as P. atromarginatus and P. petasatus have similar colored caps and also occur on woody substrates, but are distinguished by free gills that are pinkish at maturity; similar colored Tricholomas differ in lacking a lignicolous habit and often have a farinaceous odor; Mycenas, like M. overholtzii and M. galericulata, are also found on or near wood but lack the broad, well-spaced gills of Megacollybia fallax and differ microscopically in possessing amyloid spores.