Anthropologists for the Wildlife Conservation Society in New York failed to find the monkey, a Miss Waldron's red colobus, in a six-year series of forest surveys in Ghana and Ivory Coast that ended in 1999.
The determination was not a surprise to wildlife experts. The last confirmed sighting of a Miss Waldron's red colobus was more than 20 years ago in the rainforest of Ghana. It was listed as endangered in 1988.
Scientists blame its demise on hunting by humans and the disruption of habitat by logging, roadbuilding and farming.
A study documenting the extinction was published in the October issue of the journal Conservation Biology.
The Miss Wadron's red colobus was discovered in 1933. There are five more varieties of red colobus, all of which are rare.
The monkeys are popular targets for hunters who supply local markets with game meat because they weigh 20 pounds or more and live in large, noisy social groups.
"If red colobus were present near any of our survey routes, it is likely that we would've detected them," said Hunter College anthropologist John Oates, who coordinated the searches.
"The extinction may be the first obvious manifestation of an extinction spasm that will soon affect other large animals in this region unless more rigorous protection is applied immediately."
New estimates suggest that 10 percent of the world's 608 primate species and subspecies on three continents are critically imperiled. An additional 10 percent might not be in immediate jeopardy but will disappear within a few decades without vigorous protection, researchers warn.