The haemagglutination-inhibition test has been used to assess the frequency of arbovirus infections in the population of Ceylon, by age-group, and also in some animals. The antigens used were: chikungunya, JE, dengue 1, 2, 3, 4, and Tahyna.The percentage of positive reactors to the group-B viruses ranged from 16.3% to 84.6% and was lower in the mountainous part of the country. Infections with the chikungunya (group-A) virus were less frequent (from 0 to 37%) and were considered to be recent infections. The prevalence of antibodies was found to increase with age.From a comparison of the antibody titres of the indigenous Ceylonese population with those of a group of immigrant Yugoslav workers it was concluded that a titre of 1:640 would represent a significant titre for a recent infection.The animals examined included goats, cattle, bandicoots, pigs and sheep; virus infections were most frequent in the bandicoots, pigs and sheep.