Cap 4-8 cm broad, convex, broadly so in age; margin at first incurved, then decurved; surface matted tomentose to appressed squamulose, sometimes faintly cracked in older specimens; color dark-brown; context white, firm, moderately thick, unchanging when injured; odor and taste mild.
Tubes 0.5-1.0 cm long, adnate, bright yellow; pores 1-2/mm, unchanging when bruised, colored like the tubes.
Stipe 4-7 cm long, 1-3 cm thick, solid, equal, subventricose, to slightly clavate, narrowed at the base; surface often yellowish at the apex and sometimes faintly reticulate, elsewhere subglabrous, pallid to pale dingy buff.
Spores--Halling: "(10.5) 12-13.5 x 3.75-4.5 µm, subfusiform and inequilateral in profile, often with a conspicuous suprahilar depression, elliptical to fusiform elliptical in face view".
Solitary to grouped under oaks (Quercus); in our area under coast live oak (Quercus agrifolia); fruiting from after the fall rains to mid-winter.
This handsome, uncommon bolete is characterized by a dry, dark-brown cap and bright yellow pores. Its relatively small size and oak habit are also important identifying characters. According to Halling, Boletus citriniporus closest relative in California is B. flaviporus. The latter also has bright yellow pores, but can be distinguished by a viscid, reddish-brown to cinnamon-brown cap.