Summary:
Section Bivelares. This is the cultivated mushroom sold in supermarkets. Features include white cap usually with flat, brown fibrils that break up into scales; slightly reddening flesh; free, close, pinkish or pale brown gills (when young); a well-developed intermediate wedge-shaped ring; growth in rich soil or manure (normally not grass); chocolate- or violet-brown spore deposit; and 2-spored basidia (the Californian var. burnettii is 4-spored).

Cap:
3-16cm across, convex becoming flat or slightly depressed, margin inrolled when young, often extending past gills; white, usually with flattened pale brown to brown fibrils that break up into scales; dry, (Arora), 4-16cm across, at first hemispheric with strongly inrolled margin, finally nearly flat or depressed centrally; pallid at first, soon becoming light to medium brown, rarely darker brown, background color whitish to pale vinaceous; dry, at first innately fibrillose, later with appressed-fibrillose fine scales (about 0.2-0.8cm broad), or occasionally remaining loosely interwoven (for example if beneath duff or soil), (Kerrigan), margin appendiculate (Courtecuisse)

Flesh:
thick, firm; white, usually becoming brownish to reddish or pinkish orange when cut and rubbed repeatedly, (Arora), up to 2.5cm thick, moderately firm; white, usually somewhat rufescent [becoming somewhat reddish] near gills and cuticle, in stem white, slightly to moderately rufescent [becoming reddish], (Kerrigan)

Gills:
free when mature, close; pinkish or pale brown, becoming purple brown to chocolate brown when old, finally blackish brown, (Arora), free, close, up to 1.2cm broad; often pinkish when young, when old dark blackish-brown, margin pallid, (Kerrigan(1))

Stem:
2-8cm x 1-3(4)cm, equal or enlarged at base, firm, usually stout; white, sometimes turning dingy brownish when old; "smooth or slightly cottony-scaly below ring", (Arora), 3-8cm x 1-3cm, equal to slightly bulbous, stuffed-hollow; white, becoming slightly reddish-orange when cut; bald, (Kerrigan(1))

Veil:
membranous, cottony, white, 2-layered, forming delicate, median to superior ring which may collapse when old, ring intermediate or sometimes skirt-like, upper surface often striate, (Arora), veils forming thick, wedge-shaped, median, white ring, striate on upper surface, lower surface loosely interwoven, entire or grooved between partial veil and universal veil, or in high humidity partial veil long remaining attached to cap margin and expanding to appear pendant, the universal veil then pendant from partial veil, appressed to stem or not, or on rare occasions universal veil and partial veil peronate, (Kerrigan(1)), "forming a wedge-shaped, band-like, supramedian, intermediate annulus, commonly sheathing upwards but not reaching the stipe apex, rarely also an appressed pigmented basal ring, and sometimes also velar patches on the pileus (e.g., disc)", (Kerrigan(3)), the undersurface with wide toothed flaps that separate from surface and point down the stem, (Chariton)

Odor:
mild or faintly fruity (Arora), fruity/spicy after exposure, (Kerrigan(1)), pleasant (Lincoff), fungoid (Isaacs), ''familiar as "mushroom" of the market'', (Kerrigan(3))

Taste:
pleasant (Lincoff)

Microscopic spores:
spores 5.5-8.5 x 4-6.5 microns, elliptic, smooth, basidia mostly 2-spored, (Arora), spores (4.9)6.3-7.6(9.0) x (4.5)4.9-5.9(7.1) microns, broadly elliptic to elliptic, dark brown, hilar appendix often prominent, germ pore not evident; basidia predominantly 2-spored, 17-22 x 4-5 microns, cylindroclavate; cheilocystidia forming a continuous band, 20-35 x 6-10.5 microns, clavate or clavate-truncate, (Kerrigan(1)), spores 5.9-7.7(8.3) x 5.3-6.5 microns, round to broadly elliptic or obovate in both views, apiculus short, blunt, colorless, one to several droplets; basidia 2-spored, 19-25 x 6.5-9 microns, clavate to broadly clavate, with a broad pedicel, colorless, sterigmata 3.5-7 microns long, aculeate to narrowly acuminate; cheilocystidia forming nearly sterile edge, clustered, or scattered with occasional basidia, 12-32 x 7-12 microns, clavate, cylindric, broadly clavate or occasionally spheropedunculate, cylindrofusoid or saccate, often repent, colorless, (Isaacs)

Spore deposit:
chocolate brown (Arora) cocoa-brown or violet-brown (Lincoff)

Notes:
Agaricus bisporus has been identified as such in BC, WA, ID, CA, and probably more widely in the Pacific Northwest, at least as an escaped species. Kerrigan(3) says (with Latin names italicized) that it "occurs in at least three or four isolated native populations in North America. The var. bisporus occurs in coastal California (with Monterey Cypress), Alberta (with White Spruce), and New Mexico (in mixed subalpine forest). The var. burnettii is known only from desert California (with Mesquite). Several genetic markers have been used to confirm the unique, native character of three of these populations. One collection with native marker alleles is also known from Saskatchewan, Canada.". Kerrigan(3) also says that east of the central plains only European strains, surely escaped from cultivation, have been found in recent times. Breitenbach(4) give the distribution as North America, Europe, Asia, and North Africa. Kerrigan(3) says it is best known "from Europe into North Africa and from North America" but also reported from Argentina, China, Australia, Tasmania, and New Zealand.

EDIBILITY

the most widely eaten and cultivated mushroom, (Arora)

Synonyms and Alternate Names:
Agaricus brunnescens Peck (as lectotypified by Kerrigan 2008)