A seroepidemiological study of Japanese encephalitis (JE) in Dimapur, Nagaland was carried out following an outbreak of the disease between July, 1985 and February, 1986. Altogether 50 persons were affected with 30 (60 per cent) deaths. The attack and death rates per 1000 were more in Nagas viz. 0.55 and 0.34 than non-Nagas viz. 0.33 and 0.20 respectively. All ages and both sexes were affected. Of the nine mosquito species encountered Culex vishnui showed the highest density (44.5/MH). Culture of mosquito pool did not yield any viral agent. A total of 311 serum samples comprising 95 humans, 166 animals and 50 birds were tested for the presence of haemagglutination inhibition (HI) antibodies against Chikungunya and three flavirus antigens, viz. JE, WN and DEN-2. The overall flavivirus HI antibody in humans was positive in 26 (27.3 per cent) almost identical to JE antibody prevalence. The per cent positivity of HI antibodies to JE, WN and DEN-2 were 42.2 per cent, 22.2 per cent and 13.3 per cent in the affected and 14.2 per cent, 10 per cent and 6 per cent in the unaffected area. The activity of Alpha (Chikungunya) virus though very low was significantly higher (chi 2 = 5.57) in the affected area. The prevalence of JE antibody was 77.7 per cent in dogs, 52 per cent in cattle, 34 per cent in pigs and 21.1 per cent in goats. Of the five species of birds, flavivirus and JE antibodies were detected in 21.4 per cent pigeons and 22.2 per cent heron egrettes. Neutralisation test established the distinct role of JE virus over other related flavivirus antigens.