Cap 7.0-14.0 (17) cm broad, convex, expanding to plano-convex; margin incurved, later decurved to nearly plane, not overlapping the tube layer; surface moist, more or less glabrous when young, becoming irregularly pitted or wrinkled; color at first unevenly buff-brown to pale chestnut-brown, overlain initially with a whitish bloom, in age becoming medium-brown to dark-brown, subviscid when moist; context up to 2.0 cm thick white, unchanging, firm in youth, soft at maturity, tinged pinkish-vinaceous below the cuticle, sometimes yellowish above the tube layer; odor and taste mild.
Pores up to 3/mm when young, stuffed, approximately 1/mm in age, whitish, becoming cream-colored to dull pale-yellow, eventually dingy yellowish-olive, not bluing, darkening slightly where handled; tubes up to 2.0 cm long, occasionally discoloring brownish when cut, not bluing, depressed at the stipe.
Stipe 7.0-13.0 cm long, 3.0-4.0 cm thick, solid, clavate to ventricose in youth, subclavate to equal at maturity; surface of apex reticulate, whitish, elsewhere glabrous to faintly wrinkled; context of stipe, not bluing, but darkening slightly when cut; partial veil absent.
Spores 11.5-13.5 x 3.5-4.5 µm, smooth, thin-walled, narrowly ellipsoid in face-view, hilar appendage inconspicuous, one to several guttules; spore print dull olive-brown.
Solitary to scattered in mixed hardwood/conifer forests; fruiting shortly after the fall rains.
Edible and very good.
Boletus regineus is a close relative of Boletus edulis (King Bolete, Porcini). Although usually smaller and less common than its better known cousin, Boletus regineus is considered by many mycophagists to be equal in quality as table-fare. It is distinguished from the King Bolete by habitat preference--mixed hardwood/conifer woods in contrast to mostly pines for Boletus edulis, a whitish bloom in youth, and a more equal stipe at maturity. Boletus edulis differs additionally in having a cap margin that slightly overlaps the tube layer. Another edible, robust bolete found in mixed woods is Boletus appendiculatus, (Butter Bolete). This rusty-brown to yellowish-brown capped bolete gets its common name from a yellowish, often reticulate stipe, and yellow pores that blue rapidly when bruised.
Until recently we used the name Boletus aereus for our California material, although it has long been thought that Boletus aereus is a similar, but distinct, species from Europe. We now have a correct name to place on our species.