A serologic study is made in two population groups in the Isle of Youth. A total 268 blood samples in blotting paper are subjected to the hemagglutination inhibition technique, using the Eastern equine encephalomyelitis, Western equine encephalomyelitis, Saint Louis encephalitis, and dengue 2 viruses; 16% positivity to flavivirus was found. A second serum sample was taken in people positive by the hemagglutination inhibition technique in order to carry out the techniques of complement fixation and plate reduction neutralization. Nine cases showed complement-fixating antibodies, which is indicative of recent infection and in 29 cases neutralizing antibodies to SLE virus were found.