Thirty-four samples representative of each of the major aquifers and springs in the Bet Shean-Harod Valleys were analyzed for their uranium isotopes. Whereas there is relatively small variation in the uranium content, large differences are exhibited by the 234U 238U activity ratios. Three distinct water sources are delineated. The higher degree of chemical weathering in the basalt aquifer yields low activity ratios. It is proposed that the difference in activity ratio within the carbonate waters is due to isotopic changes related to the degree of water maturation which itself is related to the distance the average water travels from the recharge area to the outlets.