Cap 6-11 cm broad, convex, the margin incurved, becoming broadly convex in age, the margin then wavy; surface viscid, innately fibrillose to reticulate, cinnamon-brown to reddish-brown, shading to a slightly lighter margin, darker overall in age; flesh white, moderately thick, not bruising blue; odor mild; taste mild to sour.
Tubes slightly depressed next to stipe; pores bright yellow, approximately 1 mm broad, not bruising blue.
Stipe 6-12 cm, 1-2 cm thick, solid, equal to tapering downward or enlarged at the base, sometimes slightly rooted with adhering whitish mycelium; surface viscid when moist, smooth to uneven, white to flushed reddish-brown, the apex yellow, lined or slightly reticulate from tube remnants; flesh white, unchanging.
Spores 12-15 x 5-6 µm, smooth, elliptical to spindle-shaped; spore print olive-brown.
Solitary to scattered under oaks and madrone; fruiting from late fall to mid-winter.
This handsome mushroom with a viscid, reddish-brown to mahogany-brown cap and brilliant yellow pores, is an exception to the rule that Boletus species have dry caps and Suillus species (slippery jacks) are viscid. The often bent stipe, yellow at the apex from tube remnants, and white mycelium at the base, are also important field characters. Boletus flaviporus is know only from California.