A study was made of the occurrence of I131 in human fetal thyroid glands apparently resulting from I131 received by the mother from nuclear weapons testing or other types of population contamination. Radioactivity compatible with I131 was found in the human fetal thyroid gland. The maximum concentration of I131 was 265 μμc per gram of fetal thyroid tissue, 29.1 μμc per gram of adult hog thyroid tissue and 22.9 μμc per gram of adult thyroid tissue. The highest activity found on a reagent blank was 10.0 μμc The calculated maximum total radiation delivered to any human fetal thyroid gland was 0.47 rads. The average total dose was 0.05 rads. Extrapolations from data on animals suggest that it is unlikely that this quantity of radiation would produce hypothyroidism or carcinoma of the thyroid gland.