Serological prevalence of Flaviviridae was studied on Reunion Island by testing 2,507 human sera from a randomised sample. Each serum was tested against 5 viruses (yellow fever, dengue type 1 and 2, West Nile and Wesselsbron) using haemagglutination inhibition test: 42.68% of human sera were found positive. The multivalent reactions represent practically three fourths of the positive ones. A severe dengue type 2 outbreak on the island in 1977-1978 and the possible circulation of a Flavivirus may explain these data. Among the tested subjects, positivity frequencies vary according to their age and their living surrounding. These results were compared with those of a previous serosurvey performed in 1971 and showed an important increase in the serological prevalence largely explainable by the 1977-1978 epidemic.