Chikungunya (CHIK) virus is prevalent throughout Southeast Asia and Africa. It has caused numerous large outbreaks in India. No active or passive surveillance has been carried out since the last epidemic occurring in 1971. During a recent outbreak of Dengue (DEN)-like illness in eastern India, Aedes aegypti mosquitoes collected from the affected area were positive for CHIK virus. Evidence of dual infection with CHIK and DEN typel virus was also obtained. A widely circulating low-virulent CHIK virus is a possible explanation for the epidemiological pattern of the CHIK virus disease in this region.