They are the smallest Old World Monkey. Head and body length is 320-450 mm and tail length is 360-525 mm. Weights of males are about 1250 grams and females about 760 grams. The pelage is a greenish grey or black to greenish yellow or buff. Underparts are white or greyish white. The face is bare except for a few black hairs on the upper lip which is sometimes yellow. The cheeks have yellow fur tipped with black and there is a black line which extends from the eye half the distance to the ear. The eyes are black as well and are surrounded by yellow or orangish skin. The fur at the top of the head is black suffused with ochre and a mane of white extends across the neck and chest from the base of one ear to the other. The outer limbs are a pale yellow tinged with red and the hands and feet are similarly colored. The tail is grayish brown or black above and yellowish bellow with a blackish tip. Variations among individuals are from an overall olive to a true green tint.

They appear to prefer rainforests along rivers. The main diet consists of fruits and insects.

They are apparently non-territorial, with adult males acting as leaders during the day and sentinels at night. Adult females outnumber adult males and for a distinct subgroup with their infants and juveniles up to one year of age. Gestation is from 158-166 days and the infants are about 230 grams at birth. Sexual maturity is reached at age 4.5 in females and about 1-2 years later in males.

They are found in Liberia; Cote d'Ivoire; Nigeria apparently to the Itimbiri River in NW Dem. Rep. Congo, Central African Republic; Rio Muni and Bioko (Equatorial Guinea).