Since it was first isolated in 1956, respiratory syncytial (RS) virus has been found in many parts of the world, generally in association with human respiratory illness, especially of young children. Previous studies have examined the frequency of serological evidence of infection in certain countries. The present study is a more general survey, dealing with the prevalence of RS virus antibody in sera from 14 different countries widely separated geographically. There was evidence of infection in all the populations studied, and the presence of antibody could not be accounted for by a relationship between RS virus and measles virus. It would therefore seem probable that RS virus is an agent of world-wide importance.