This paper describes a focus of Japanese encephalitis in Modhupur Forest area, Bangladesh. The disease was diagnosed for the first time in Bangladesh. The diagnosis was based on the clinical picture of the disease and the results of serological investigations; the causative agent could not be isolated. During 2 year's surveillance of the focus, no new cases of the disease were recorded. Only 1.9% of 1046 healthy persons tested showed HI antibodies against group B arboviruses, and no virus was isolated from 41 mosquito pools. These results indicate that the disease was introduced into the area, produced an outbreak and later died out.