Thyroidal iodine uptake was measured in fish from fresh-water habitats in northern Michigan, and from fresh and brackish waters of Long Island, New York. Michigan species, Percina caprodes, Notropis deliciosus and Umbra limi were injected with 20 p.c of carrier-free 1’31, kept in running lake water, and sacrificed at intervals up to 7 days after injection. In Percina and Notropis the maximum uptake, 20% to 30% of the injected dose, occurred 16 hours and in Umbra 50 hours after injection. Iodine loss was slow and one-third of the accumulated isotope was still in the thyroid after 5 days. The temperature of the water had a profound effect on the rate of thyroidal iodine uptake. When Umbra limi and Fundulus diaphanus were kept in cold well water, the observed maximum thyroidal accumulation, 20% of the injected radioiodine, was found 6 days post injection, and was still rising when the experiment was terminated. In New York species Lepomis gibbosus (fresh water), Cyprinodon variegatus and Fundulus heteroclitus (brackish water), 1’31 was injected and animals sacrificed as above. Experimental fish were maintained in water from their original habitat. In Lepomis the maximum uptake of 17% was reached rapidly and loss was slow. In different tests of thyroidal 1’31 uptake by Fundulus and Cyprinodon, maxima of approximately 5% to 15% were found. These differences seemed to be related to the salinity of the brackish water in which the fishes were kept, being in some instances almost fresh, and in others quite salt.