Cap 4-8 cm broad, convex, broadly convex to nearly plane in age; surface white, dry, innately fibrillose, finely-scaled at the margin, discoloring brown in age; flesh, white, moderately thick, firm, turning pinkish-red when injured; odor pungent of "mushrooms", taste mild.
Gills free, close, pinkish-brown becoming blackish-brown in age.
Stipe 5-11 cm tall, 1-2 cm thick, equal, to enlarged at the base, stuffed at maturity; surface white, more or less smooth at the apex, finely scaled below; partial veil white, membranous, two layered, upper surface striate, lower surface with scaly patches, forming a small, superior annulus; flesh white, staining red quickly when injured.
Spores 5-6 x 3-4 µm, smooth, elliptical; spore print blackish-brown.
Solitary to scattered under Monterey cypress (Cupressus macrocarpus) and pines (Pinus); fruiting from mid to late winter.
Agaricus benesii is distinguished by a white cap which bruises pinkish-red when injured, a scaly lower stipe, and conifer habitat. In age the cap may discolor brown creating confusion with Agaricus californicus or A. xanthodermus. These species, however, have an annulus with a thickened margin, a phenolic odor, and bruise yellow, not red (quickly in the case of Agaricus xanthodermus and faintly or not at all in the case of A. californicus. Agaricus bernardii is also white and stains red, but is a larger, much stouter mushroom with a sheathing veil, a briny odor, and typically is found in grass.