In a serological and entomological survey on yellow fever carried out in Sierra-Leone in 1972, altogether 899 sera from children 0 to 14 years were tested with 12 antigens by haemagglutination-inhibition and complement fixation tests. Mouse neutralization test with yellow fever, West-Nile and Zika viruses were also performed on selected sera. Generally speaking, the incidence of arboviruses is low but the prevalence of antibodies for some viruses was found to vary considerably between different areas. As regards yellow fever, the virus has recently been in circulation in only two areas: Bafodia and Lalehun-Labour Camp and there is no risk for a yellow fever outbreak to occur in the near future. Due to the shortness of the survey, entomological prospections were confined to a search for Ae. aegypti larvae in and around dwellings: no breeding places are found in houses and Breteau indices are usually low, especially in forest villages. On the other hand, in urban settlements in the mining areas, breeding places around houses are numerous and are bound to increase in number. All the conditions necessary for the outbreak of an epidemic would be present within few years: such a situation would appear in Labour Camp where yellow fever virus has been circulating, where most of the population has no immunity and where Breteau indice goes as high as 34.4. As regards the other arboviruses, Zika virus is active in most areas and Chikungunya virus is particularly active in the plateau and savanna zones, in the North-East.