Anomalously high levels of naturally derived uranium have been found in groundwater samples from wells completed in overburden and bedrock aquifers in southeastern Manitoba. Uranium analyses were carried out on groundwater samples collected from 287 individual wells in a 15 500 km2 area. The mean U concentration in all samples was 58.3 μg/L; the maximum value was 2020 μg/L. Uranium concentrations were highest in samples from Precambrian rock aquifers, averaging 115.6 μg/L, and were lowest in samples from Paleozoic sedimentary rock aquifers, averaging 3.5 μg/L.Uranium concentrations up to 250 μg/L were found in groundwaters in or associated with Lake Agassiz clay deposits. These high concentrations are thought to result principally from release of U from the organic component of the clays, either by oxidation or desorption. Leakage of uraniferous groundwater from the clays appears to provide a significant source of U to underlying sand and gravel aquifers and to the Precambrian bedrock aquifer. Uranium within the bedrock is also being released to solution by oxidation of uraniferous hematite found as mineral coatings and fracture fillings in highly weathered zones. Uranium solubility in many Precambrian bedrock groundwaters is increased by recharge of these aquifers by ground waters from overburden materials that contain high concentrations.