Cap 7-16 cm broad, convex at first, margin incurved, becoming plane, disk frequently depressed at maturity; surface white, smooth, though typically with warts or cottony scales. Cap tissue turning reddish-orange when cut. Odor, of brine or iodine.
Gills free, close, pinkish-tan becoming chocolate brown, finally blackish brown.
Stipe 4-7 cm long, 3-4.5 cm thick, smooth, narrower at base; veil membranous, sheathing the stipe base, forming a medial ring.
Spores 5.5-7.0 x 5.5-6.5 µm, smooth elliptical. Spore print blackish-brown.
Found spring, summer and fall primarily in grassy areas, sandy soils, occasionally under Monterey cypress; gregarious sometimes forming arcs but rarely complete fairy-rings. In grass it often fruits with Agaricus californicus, A. arvensis, Marasmius oreades, and Leucoagaricus naucinus.
Edible and excellent, although the briny taste deters some people.
Agaricus bernardii is a short, stocky, white mushroom that resembles Agaricus bitorquis, even possessing a sheathing veil and a propensity for partial emergence. However, it can easily be told apart by the reddish-brown staining of cap and stipe tissue as well as its briny odor. Along the coast, particularly in sandy soils, Agaricus bernardii is considerably more common than A. bitorquis.