Serum samples from 54 monkeys were collected from healthy individuals in a monkey farm in Luzon island, Philippines, in 1999, and examined by IgM‐capture ELISA and indirect IgG ELISA for the presence of dengue (DEN), Japanese encephalitis (JE) and chikungunya (CHIK) viruses. The positive rates for IgM ELISA were 3.7, 35.2 and 14.8% against DEN, JE and CHIK, respectively. Higher positive rates were obtained when indirect IgG ELISA was used: 100% against flaviviruses (JE or DEN) and 59.3% against CHIK virus. The results indicate a high prevalence of flavivirus infections such as JE and DEN, and a lesser prevalence of CHIK virus infections, among monkeys in the Philippines. These findings suggest possible sylvatic transmission cycles of these viruses.