Field tests studying more than 600 cases verified the disease was to be found near Wamba, at the northern Oriental province.
Reports say that the scope of the infection could probably be less because of the number of deaths.
"Preliminary results from a rapid diagnosis test in the field found three samples positive out of eight," the WHO said.
Early this summer, it is surmised that pneumonic plague may have killed 100 people in eastern Congo.
The WHO reports that plague is endemic in Africa, the Soviet Union, the Americas and Asia. If not addressed quickly, it has a case-fatality ratio of 30%-60%.
