Three species of forest guenons live sympatrically in a mountain forest, Kalinzu, Uganda. Among them, the blue monkey (Cercopithecus mitis) and red-tailed monkey (C. ascanius) are arboreal, but l’hoest’s monkey (C. lhoesti) is highly terrestrial. They often form a mixed-species group to travel and feed together for a long time. In particular, blue and red-tailed monkeys make such associations almost every day. Though forest guenons are generally considered frugivorous, all in this forest spend a very long time searching for and eating invertebrates. Little is known about how these congeneric species coexist and why guenons living in this forest feed for hours on insects. Firstly, I observed them and collected 156 faeces from August to September in 2016. Blue monkeys consumed more kinds of plant parts and ate a wider variety of insects than the other species. For both herbivory and insectivory, blue and red-tailed monkeys commonly used more items than the other two combinations. To reveal the consumed insect fauna, I extracted DNA from faeces, amplified the COI region of mtDNA using insect-specific primers, sequenced the amplifcations with Illumina MiSeq, and compared the obtained sequences to those in reference databases (DNA metabarcoding). Supporting the behavioral observation, blue monkeys consumed more varied insects than the others, and blue and red-tailed commonly ate more items than the other two combinations. These results consistently suggest that blue monkeys have the widest niche breadth, and blue and red-tailed occupy very similar niches even when they form a mixed-species group.