Marked differences in accumulation of I131 in milk and people were recorded during July and August, 1962, along fallout trajectories with origins at the Nevada Test Site. Iodine131 was measured in milk from thirty-nine dairy farms situated in geographically and ecologically different areas of Utah, and in twenty-four persons residing on these farms. Total I131 intake during this incident, assuming consumption of one l. milk/day, ranged from 0 to 800,000 pc. Total body and thyroid burdens of people from farms paralled levels in milk produced on their farms. Accumulation in milk was correlated with feeding practices; highest accumulation was found in milk and people from farms where cattle were fed freshly-cut green alfalfa or were grazing on wet meadows. Little I131 appeared in the milk of cattle fed uncontaminated feed which had been stored prior to the arrival of fallout. Highest I131 levels occurred on farms at high altitudes.