This paper describes the results of a study of antibodies to several arboviruses in representative groups of children in Portuguese Guinea. The survey was conducted in late 1964 and early 1965, in conjunction with a yellow fever vaccination programme undertaken when this disease appeared in West Africa. Sera were prepared from 1103 blood specimens collected from 10-15-year-old residents in different regions of the territory and were tested against 17 different antigens by means of the haemagglutination-inhibition test. In addition, 51 sera obtained from 20-25-year-old nonresidents were tested. The results show that group B arboviruses, particularly the yellow fever virus, were active in the region. Information on the activity of arboviruses of other groups was also obtained.