Abstract 1. 1. Nereis diversicolor contains between 23 and 30 μg of iodine per g. 2. 2. Iodide is absorbed at about 5 ng/g per day under conditions approximating to the normal environment. 3. 3. Most of this is subsequently lost in mucus secreted, but 15 to 30 per cent becomes bound in the basal regions of the chaetae where tanning is occuring. 4. 4. The chemical forms of the bound iodine have been examined and found similar to those of other invertebrates. 5. 5. The method of binding is discussed.