Waterfowl were collected at radioactive leaching ponds, an adjacent sewage lagoon on the Idaho National Engineering Laboratory ( INEL ) and on nearby control areas to determine 129I / 127I (stable isotope) atom ratios in muscle tissue. Iodine-129/127 atom ratios in muscle tissue averaged about 1.6 X 10(-6) for wild waterfowl from the leaching pond, 1.6 X 10(-6) for waterfowl from the sewage pond, and 1.3 X 10(-6) for control samples. A group of wing-clipped waterfowl placed on the ponds for 2-156 days had average 129I / 127I atom ratios in muscle of 6.0 X 10(-6). Iodine-129/127 atom ratios in muscle were statistically higher (P less than 0.05) in the wing-clipped waterfowl than in control, sewage lagoon and wild waterfowl samples because of the extended period of time they spent on the ponds. Internal lifetime whole-body dose from 129I was estimated to be 1.0 X 10(-5) mrad for waterfowl from control areas and 3.0 X 10(-5) mrad for leaching pond waterfowl. The lifetime thyroid dose commitment to man consuming a wild duck from the leaching ponds was 1.3 X 10(-4) mrem. Liquid effluents disposed in the radioactive leaching ponds do not appear to cause an appreciable increase in 129I / 127I atom ratios in tissues of waterfowl using the ponds.