Acute Encephalitis Syndrome (AES) and Japanese Encephalitis (JE) is a major public health problem in Bihar leading to large scale morbidity and mortality especially among the childrens. Recurrent outbreaks of AES/JE have been reported in Muzaffarpur district, Bihar in recent years. Outbreak of AES/JE in Muzaffarpur in 2016 was investigated. Epidemiological, Entomological and Environmental investigations were carried out. From 5 Jan 2016 till 14 July 2016, total 70 cases and 7 deaths due to AES were reported in Muzaffarpur district. Out of the total reported cases, etiological confirmation could be done in 24% of the cases while etiology could not be confirmed in 76% of the cases. Out of the total confirmed cases, JE contributed 81% of the cases while the rest were confirmed as pyogenic and tubercular meningitis. Block Mushahri was the most affected followed by block Minapur. Fever, altered sensorium & seizures were present in 100% of the cases. Outbreak peak was observed on 1-10 June 2016. Per Man Hour Density (PMHD) of culex was higher when compared to other mosquito species. Culicine Larval density/dip was higher in cattle feeding containers. Most of the affected population belonged to low socio-economic strata. Awareness regarding the disease was also very less in the affected population. The study would help to identify the factors leading to the recurrent outbreaks in the region and for undertaking appropriate remedial actions.