Kingdon (1997), Grubb et al. (2003), Groves (2005), and Cords and Sarmiento (2013) recognise five subspecies: C. a. ascanius, C. a. atrinasus, C. a. katangae, C. a. schmidti and C. a. whitesidei.

This species is listed as Least Concern because it is locally common with an extensive geographic range, where it occupies a wide variety of habitats, including forest islands, degraded forests and secondary forests. This species can withstand some hunting pressure and, as a relatively small-bodied monkey, it is not a preferred target of bushmeat hunters.

The Red-tailed Monkey ranges from south and east of the Congo-Ubangui River system in Central Africa, through Uganda into southwest Kenya to the west flank of the eastern (Gregory) Rift Valley, north and west Rwanda, west Burundi, west Tanzania, westwards through extreme north-west Zambia, to central Angola (Gautier-Hion et al. 1999; Butynski 2002a; Groves 2001, 2005; Grubb et al. 2003; Cords & Sarmiento 2013). It occurs up to 2,500 m asl.

Five subspecies are recognised:

Cercopithecus a. ascanius (Black-cheeked Red-tailed Monkey): occurs in northern Angola and southwestern DRC, south of the Congo and Kasai Rivers below 500 m asl. North of this, some animals between the Kasai River and Lukenie River may be attributable to this subspecies (Sarmiento et al. 2001). The eastern and southern extent of its range are poorly known. The subspecific status of a population of C. ascanius in northern central Angola is not known, but is thought to be either C. a. ascanius or C. a. katangae (Sarmiento et al. 2001, Cords and Sarmiento 2013).

Cercopithecus a. atrinasus (Black-nosed Red-tailed Monkey): endemic to northeastern Angola, this subspecies has a small geographic range (ca 2,500 km², Cords & Sarmiento 2013) on the Lunda Plateau around Wamba and Lué Rivers. Type locality Zova (850 m asl). Although the type locality is near DRC, this subspecies has never been recorded from that country (Sarmiento et al. 2001).

Cercopithecus a. katangae (Katanga Red-tailed Monkey): occurs in extreme northeastern Angola, extreme northwestern Zambia, and southern and central DRC, west of the Lualaba River as far north as the Lualaba River and Lomami River junction. In the west, its northern limit is the junction of the Sankuru and Kasai rivers. The western limit in Angola is the right bank of the upper Kwilu (Cuilo) River. Occurs between 500 and 1,300 m asl. The subspecific status of a population of C. ascanius in northern central Angola is not known, but is thought to be either C. a. ascanius or C. a. katangae. See Sarmiento et al. (2001) and Cords and Sarmiento (2013) for further details of its distribution.

Cercopithecus a. whitesidei (Congo Basin Red-tailed Monkey): endemic to central DRC, mainly south and east of the Congo River, west of the Lomami River. The Lukenie River marks the southern boundary, although individuals with a similar phenotype are known from south of the Lukenie River (Sarmiento et al. 2001).

Cercopithecus a. schmidti (Schmidt’s Red-tailed monkey): occurs east of the lower Ubangi River and Lualaba River, north of the Congo River and east of the Lualaba River in DRC. Eastwards through southern Uganda (northern limit Victorian Nile), Burundi, Rwanda, central western Tanzania and southwestern Kenya (west of Eastern Rift Valley). Isolated populations are found in central northern Central African Republic and southwestern South Sudan (Sarmiento et al. 2001, Cords & Sarmiento 2013). Altitudinal range is between 400 and 2,500 m asl, but mainly >1300 m in the eastern part of its range (Sarmiento et al. 2001).

Transitional forms occur between whitesidei and katangae, katangae and ascanius, and katangae and atrinasus. There is a wide transitional zone between ascanius and atrinasus (Lernoud 1988, Sarmiento et al. 2001, Cords and Sarmiento 2013). Cercopithecus ascanius hybridizes with other species, including Cercopithecus mitis stuhlmanni, Cercopithecus mitis doggetti, and Cercopithecus denti (Struhsaker et al. 1988, Detwiler et al. 2005, Davies 2013).

This species is locally common throughout its range. Densities of C. a. schmidti are 8-184 ind./km² (1.0-13.3 groups/km²; Cords and Sarmiento 2013). In Budongo Forest, Uganda, densities higher in heavily logged forest (46-60 ind./km²) compared to unlogged forest (8-19 ind./km²; Plumptre and Reynolds 1994, Sheppard 2000). These findings contrast with those of Chapman and Lambert (2000) for Kibale NP, Uganda, who report highest group densities (4.8-11.5 groups/km²) in lightly logged forest, intermediate densities in unlogged forest (3.8-5.6 groups/km²) and lowest densities in heavily logged forest (1.0-2.5 groups/km²).

Cercopithecus ascanius is an arboreal frugivore-insectivore that also feeds on young leaves. The species is found in moist lowland, submontane and montane forests, swamp, riverine and gallery forest, and forest mosaic. It occurs in secondary or regenerating forests, forest islands and plantations, and prefers forest edges. It is absent from the interior of primary forest (Cords and Sarmiento 2013). 

Typical group size is 25–35 individuals (Cords and Sarmiento 2013).

Locally threatened by habitat degradation, loss and fragmentation, and agricultural encroachment. The rapidly growing human population in some parts of the range has led to habitat destruction related to farming, timber and fuelwood extraction, dam, road and powerline construction, mining, exotic tree plantations, human settlements, and so on (Butynski 2002b, Cords and Sarmiento 2013). Although C. ascanius is one of the least attractive African primates to hunters (alert, fast, small body size), the species is affected locally by retaliatory hunting in response to crop raiding, and commercial bushmeat hunting (Butynski and Sanderson 2007, Cords and Sarmiento 2013).

This species is hunted for the commercial bushmeat trade. It is also hunted in retaliation for crop raiding (Butynski and Sanderson 2007, Cords and Sarmiento 2013).

Cercopithecus ascanius is listed on Appendix II of CITES and on Class B of the African Convention on the Conservation of Nature and Natural Resources. This species is present in many protected areas across its wide geographic range including Mahale Mountains NP and Gombe NP (Tanzania), Kibale NP, Queen Elizabeth NP, Semliki NP, Ruwenzori NP and Bwindi Impenetrable NP (Uganda), Kakamega NR and Masai Mara NR (Kenya), Nyungwe Forest NP and Volcanoes NP (Rwanda), Bururi Forest NR (Burundi), and Kahuzi-Biega NP, Tayna Gorilla Reserve, Okapi Wildlife Reserve, Maiko NP, Virunga NP, Garamba NP and Salonga NP (DRC).

There is an urgent need to assess the current geographic distribution and conservation status of C. a. atrinasus.