The specific epithet fusca is derived from the Latin word fuscus, meaning "dark brown" or "dusky", and refers to the colour of the pitchers.

According to Endert it grew in the forest on a narrow, stony mountain ridge covered with humus, and was not rare.Botanist Jan Schlauer has noted differences between the type specimen of N. fusca and Sabah plants referred to this species,[6] even interpreting plants illustrated in Kurata's Nepenthes of Mount Kinabalu as representing N. stenophylla (as distinct from N. fallax).

He suggests that the type specimen consists of intermediate lower and upper pitchers as opposed to true forms of either, making them appear atypical.

[6] Much of this taxonomic uncertainty stems from the fact that N. fusca has not been recollected from the type locality and many similar plants have been lumped under this taxon.

[8] Matthew Jebb and Martin Cheek attempted to resolve this confusion in their 1997 monograph by interpreting N. fusca as a widespread and variable species.

[11] Two subspecies of N. fusca have been described, neither of which is presently thought to represent the species: Both were originally coined by J. H. Adam and C. C. Wilcock and subsequently published in Jebb and Cheek's 1997 monograph, "A skeletal revision of Nepenthes (Nepenthaceae)".

[6] Matthew Jebb and Martin Cheek resolved this confusion in their 1997 monograph by referring a number of Bornean plants identified as N. maxima to N. fusca, thereby excluding the former from the island.

[citation needed] The authors described the taxon as a Sabah endemic growing at elevations of 1200 to 1500 m.[citation needed] Adam and Hafiza wrote that N. zakriana "consistently differed from Nepenthes fusca by prominent raised midribs, extended beyond apex forming an apical glandular appendages on lower lid surface of both upper and lower pitchers; and basal half portion of the midrib developed in nail-shaped glandular crest".

[citation needed] However, a number of authors soon voiced their doubts as to whether N. zakriana merited species status[16] and in Pitcher Plants of Borneo by Anthea Phillipps, Anthony Lamb, and Ch'ien Lee, these features were considered to fall within the natural variability of N. fusca.

[17] The pitchers of this plant match J. H. Adam and C. C. Wilcock's description[18] of N. faizaliana,[6] but the latter is now known to have a round lid (as opposed to narrowly triangular in N. fusca and "Nepenthes sp.

[6] Clarke proposes that this taxon might fall under Jebb and Cheek's more inclusive concept of N. fusca, but retains it as an undescribed species because it remains poorly known.

The petiole (≤4 cm long)[5] is grooved lengthwise and bears a pair of narrow wings that form a semi-amplexicaul sheath around the stem.

[8] Nepenthes fusca is most commonly found as an epiphyte in shady mossy forest on ridge tops, where it may grow 10 to 15 m off the ground.

[33] The species is abundant on Mount Alab, the highest peak of the Crocker Range, where a number of colour variants have been documented, including one with wholly green lower pitchers save for red mottling on the interior and underside of the lid.

[8] Other notable locations include the Kimanis–Keningau Road that runs through the Crocker Range[8] and the summit area of Mount Apo Dari (where it grows at 1500 m).

[14] On Mount Mulu in Sarawak, N. fusca (found below 1200 m) appears to occupy a discrete altitudinal zone from N. vogelii (1200–1500 m) and N. hurrelliana (above 1500 m), both of which are also epiphytes.

[8] In the Hose Mountains of central Sarawak, N. fusca is typically found growing as an epiphyte in lower montane forest at 700–1200 m.[34] It is also an uncommon inhabitant of roadside embankments there.

[34] Charles Clarke noted that since substantial populations of N. fusca lie within the boundaries of national parks, they "are unlikely to become threatened in the foreseeable future".

The upper pitchers of N. hurrelliana differ in having a horizontal mouth that rises abruptly into a long neck at the back and in having a hirsute basal crest on the underside of the lid.

In their description of the former, J. H. Adam and C. C. Wilcock distinguished these taxa on the basis of inflorescence structure, the size of the glandular region on the inner surface of upper pitchers, and the development and characteristics of the indumentum.

[8] The authors noted that both N. fusca and N. lowii are common on the summit area of Mount Alab where this plant is found, whereas N. chaniana is rare.

In addition, this hybrid has a less ovate lid, which lacks the bristles characteristic of N. lowii, and a denser indumentum covering the stem and leaves.

[12] Nepenthes naquiyuddinii is generally thought to be a heterotypic synonym of N. reinwardtiana,[16] but may also represent this hybrid, as both putative parent species grow in close proximity to it.

[8] Nepenthes fusca × N. tentaculata was discovered by Linus Gokusing near the summit of Mount Alab, where it grows in upper montane forest at elevations of around 1800 to 2000 m.[8] It is sympatric with both parent species, which are abundant in the area.

In 2004, professional horticulturist Robert Sacilotto wrote an article for the Carnivorous Plant Newsletter, summarising measured tolerances of several highland Nepenthes species based on experiments conducted between 1996 and 2001.

[44] Sacilotto found N. fusca to be tolerant of a wide range of conditions; with the exception of plants not treated with fungicides, no test groups showed a survival rate of less than 75%.

[44] An illuminance of 6400–8600 lx (600–800 fc) proved to be optimal when plants were grown under sunlight, high pressure sodium, and metal halide lamps.

However, specimens placed under an even combination of Gro-Lux and cool white fluorescent lamps at 5400–7500 lx (500–700 fc) exhibited the most vibrant colours (although growth rates remained the same).

Plants moved from the former to the latter light set up showed a significant change in pigmentation; green leaf blades turned bronzy and speckles on the pitchers darkened markedly.

[2] b.^ The Latin description of N. fusca from Danser's monograph reads:[2] Folia mediocria breviter petiolata, lamina lanceolata, nervis longitudinalibus utrinque c. 2, vagina caulis 1/2 amplectente; ascidia rosularum ignota; ascidia inferiora magnitudine mediocria, parte inferiore anguste ovata, os versus subcylindrica, parte superiore alis 2 fimbriatis; peristomio in collum elongato, applanato, 4-10 mm lato, costis c. 1/3-2/3 mm distantibus, dentibus c. tam longis quam latis; operculo anguste ovato, subcordato, facie inferiore appendice lateraliter applanata; ascidia superiora magnitudine mediocria, infundibuliformia, costis 2 prominentibus; peristomio in collum elongato, applanato, 3-8 mm lato, costis 1/3-1/4 mm distantibus, dentibus brevissimis; operculo anguste ovato, subcordato, facie inferiore prope basin appendice lateraliter applanata; inflorescentia racemis parvus, pedicillis inferioribus c. 8 mm longis, omnibus 1-floris v. partim 2-floris ; indumentum iuventute densissimum, denique passim densum, breve, e pilis patentibus crassis simplicibus v. basi ramosis compositum.c.^ Some authors treat N. fallax in synonymy with N. stenophylla,[6][11] while others consider them to be two distinct species, with plants commonly referred to as N. stenophylla actually representing N. fallax.