Blood samples taken from 1,055 indigenous residents of Trinidad, B.W.I., were tested for neutralizing antibody against 17 different viral agents known or believed to be arthropod-borne, in 4,555 protection tests. Yellow fever, Ilhéus and dengue are, or have been, on the basis of protection tests, presumably present, widespread, and common in Trinidad. A yellow fever outbreak occurred during the course of investigations in 1954. There is no evidence that Venezuelan equine encephalitis has been present on the island since the last reported outbreak in 1943. A small number of sera were found which neutralized St. Louis encephalitis virus. No interpretation of this result is attempted. Although no human sera neutralized Eastern equine encephalitis virus, two different specimens of serum from one donkey neutralized the virus in two different tests. Interpretation of tests against Uganda S, West Nile, Semliki, Ntaya, and Japanese B is complicated by the possibility of interfering cross-immune reactions. All tests with Bwamba, Zika, Anopheles A, Western equine encephalitis, and Bunyamwera viruses were negative. A provisional ecological zoning of the island is presented, based upon distribution of immunity against several virus agents.