The ultra-trace determination of arsenic in river waters was performed using the two-step preconcentration method of solvent extraction with zinc hexamethylenedithiocarbamate and back- extraction with Cu(II) solution in combination with FI-HGAAS. The 73 sample waters were collected from rivers in Iburi and northeast Oshima, Hokkaido and southwest Ghana, Africa and from Mobetsu and Horikabu rivers in Hokkaido. The total arsenic concentrations for all samples except for three samples in Ghana were less than the 10 μgl-1 limit established by WHO.