This paper discusses the significance of Strontium‐90 (Sr90) levels in the surface waters of the United States. The data represent a rather extensive study, in that samples were collected weekly from one to eight stations on each of fifteen major river or lake systems throughout the United States. Aliquots of the 1‐liter samples were taken for gross activity measurements, and equal residual portions of the weekly samples from each station were combined into a 3‐month (13‐week) composite sample of about 4 liters. These samples were then analyzed for Sr90, with the use of procedures previously described. From these values, it was calculated that the average runoff of Sr90 in these waters amounts to 4 c/day, corresponding to 0.5 mc/sq mi/yr, or 190 μμc/sq m/yr. The total gross fallout contribution was calculated as 23,900 c/day, of which 408 c/day is found in runoff. Calculations also showed that at Chattanooga the major source of Sr90 was not fallout but Oak Ridge National Laboratory operations. This condition is not found at the Hanford or Savannah River operations. Estimated rates of Sr90 runoff, from 1958 and 1959 flow data in the Ohio River Valley, showed somewhat higher levels than the average value of 190 μμc/sq m/yr. The current maximum permissible concentration level of Sr90 in drinking water for an uncontrolled area is 100 μμc/l. One‐third of this concentration has been suggested as the average concentration applicable to the entire population. The contribution to general Sr90 intake from water alone is quite small, certainly less than 5 per cent of the maximum permissible concentration. Food, however, not water, is the major vehicle of Sr90 intake. On the average, water may be responsible for 10 per cent of ingested Sr90. In Chattanooga or in places where cisterns are used as a source of supply, the Sr90 from water is nearly 35‐50 per cent of the total intake.