Slightly alkaline bicarbonate sulfate ground water, similar to modern ground water, is thought to have been the agent that formed uranium deposits in fluviatile sandstone in Wyoming. Such water has the general characteristics of the aqueous solution which investigations indicate could transport uranium at low temperatures and from which it could be precipitated. Lack of evidence for conduits in underlying rocks and absence of igneous rocks near deposits suggest that they were not formed by juvenile thermal water. The large volume of mildly oxidized rocks marginal to the deposits suggests that oxygen-poor connate water was not the agent. Modern ground water moves downward in the same direction as associated surface drainage; the ancient water which formed the deposits undoubtedly moved in a similar way. Besides being the agent for emplacement of uranium, ground water may be a guide to general areas in which deposits occur.