1. Through the use of radioactive iodine the following information concerning the removal of iodine from sea water by Ascophyllum has been established: the removal occurs as an exchange process between iodine in the sea water and iodine already present in the cells; the iodine is exchanged in the form of iodides, not iodates; the exchange takes place in living cells only and in entire plants as well as the isolated segments studied in detail.2. The exchange rate is dependent upon concentration of iodine available. Two different exchange rates may exist, suggesting that a two-step process is involved.3. The iodine concentration within the alga is about 220 times that of sea water and the absolute iodine content of the alga examined was 0.19 mg./g. fresh weight.