Sera from residents in 15 localities in the Amazon Valley were examined for antibodies to 14 arbor viruses by neutralization tests in mice. Viruses neutralized in high ratios were yellow fever, 44.7 per cent; Ilhéus, 36.2 per cent; VEE, 16.7 per cent; and Semliki Forest virus, 13.9 per cent. Viruses neutralized in lower ratios were dengue I, 9.6 per cent; West Nile, 6.1 per cent; St. Louis, 5.5 per cent; Bunyamwera, 4.7 per cent; EEE, 4.1 per cent; Zika, 4.0 per cent; Uganda S, 2.1 per cent; dengue II, 2.0 per cent and WEE, 1.6 per cent. Bwamba was the only virus tested for which no antibodies were found. The evidence is not conclusive that Uganda S, Zika, West Nile, St. Louis, dengue II, or WEE virus occurs in the Amazon Valley. The positive results obtained with these agents may be due to immunological overlap with other immunologically related agents.