A prospective serologic study was undertaken in a Rangoon community in 1982 to quantify Japanese encephalitis virus infections in humans and in pigs. The prevalence, relative abundance, and host preference of Culex mosquitoes in the area were also determined. Japanese encephalitis virus infection was detected in 52.1% of the pigs, although concurrent human Japanese encephalitis virus infection was not detected. The known Japanese encephalitis virus vector mosquito species were prevalent in the study area. Host preference determination for the Culex species revealed that they were zoophilic and that the most frequent hosts were cows. The absence of concurrent human Japanese encephalitis virus infection in the presence of Japanese encephalitis virus infection in pigs and of vector mosquito species is discussed.