We have measured dissolved nitrous oxide (N2O) concentrations in Michigan surface waters by equilibration of the water samples with ambient air followed by gas chromatographic analysis of the air. Two rivers and one lake show significant N2O supersaturations (factors of five or more) directly traceable to sewage treatment plant effluents. Municipal tap water samples from 19 U.S. cities also show supersaturations. Our data indicate that chlorination of river water and of wastes might be a key step in N2O production. To measure N2O fluxes from water to air we employ a gas collection device; two initial flux measurements from supersaturated waters are reported.