Its fruit bodies have convex to flat caps that are some shade of brown, buff or tan and typically measure up to 15 cm (6 in) in diameter.

Like most boletes it lacks a ring and it may be distinguished from Boletus edulis and other similar species by its unusual pink pores and the prominent dark-brown net-like pattern on its stalk.

The species was first described in the scientific literature as le bolet chicotin (Boletus felleus) by French mycologist Pierre Bulliard in 1788.

[1][7] Lucien Quélet placed the taxon in Dictyopus in 1886 and then Rhodoporus in 1888,[2] but neither of these genera are recognised today, the former having been merged into Boletus and the latter into Tylopilus.

uliginosus, was recognised by Alexander H. Smith and Harry D. Thiers in 1971 on the basis of its microscopic features,[2] a distinction supported by Professor C.B.

[14] Although some records exist of T. felleus in Australia, their spores are of consistently smaller dimensions and this taxon has been classified as a separate species, T. brevisporus.

[19] It is lighter in colour than the cap, and covered with a coarse brown network of markings,[19] which have been likened to fishnet stockings in appearance.

[25] Italian cook and author Antonio Carluccio reports this is one of the most common fungi brought to him to identify, having been mistaken for an edible species.

[26] T. felleus is found in the same habitat as B. badius, though the latter's yellow tubes and blue-bruising flesh easily distinguish these very dissimilar species.

[22] Tylopilus rubrobrunneus, found in hardwood forests of eastern North America, is similar in appearance to T. felleus but has a purplish to purple-brown cap.

[30] It is found in deciduous and coniferous woodland, often under beech and oak[23] in well-drained acid soils,[31] which can be sandy, gravelly or peaty.

The mycorrhizal plant partner benefits from the fungus's ability to use these forms of nitrogen, which are often abundant in the forest floor.

[32] Fruit bodies appear over summer and autumn, anytime from June to October or even November, in many of the northern temperate zones.

[36] The small fly species Megaselia pygmaeoides feeds on and infests the fruit bodies of T. felleus in North America though it seems to prefer other boletes in Europe.

[37] Infection results in necrosis of the mushroom tissue and a yellow colour caused by the formation of large amounts of pigmented aleurioconidia (single-celled conidia produced by extrusion from the conidiophores).

[38] The bacterium Paenibacillus tylopili has been isolated from the mycorrhizosphere of T. felleus; this is the region around its subterranean hyphae where nutrients released from the fungus affect the activity of the microbial population in the soil.

[41] Even when cooking, it smells terrific, but one taste of the Bitter Bolete would not only disappoint but perhaps depress the novice mushroom hunter.

[43][44] A local recipe from France, Romania and East Germany calls for stewing it in skimmed milk, after which it can be eaten or powdered and used for flavouring.

[20] Tylopilan is a beta-glucan that was isolated from the fruit bodies in 1988 and shown in laboratory tests to have cytotoxic properties[48] and to stimulate non-specific immunological response.

[49] In experiments on mice with tumour cells it appeared to have antitumour effects when administered in combination with a preparation of Cutibacterium acnes in a 1994 Polish study.

[50] Researchers in 2004 reported that extracts of the fruit body inhibit the enzyme pancreatic lipase; it was the second most inhibitory of 100 mushrooms they tested.