Conservationists announced Thursday they have discovered a large population of Phayre's langur (Trachypithecus phayrei) at the Xuan Lien Nature Reserve in Thanh Hoa Province.
The 200 individuals belong to seven different troops, according to the nature reserve management.
It did not say how such a large population remained undiscovered for so long.
Phayre’s langurs used to be found in 11 provinces in Vietnam, but their numbers have shrunk and they are only found in five now.
Earlier this month conservationists also found around 500 gray-shanked douc langurs (Pygathrix cinerea) in Kon Tum Province.
The population of gray-shanked douc langurs, found only in central Vietnam, is estimated at 800-1,000.
It is listed among the world’s 25 most endangered primates after being threatened by deforestation and poaching.