Uptake and depuration experiments for the edible periwinkle Littorina littorea have been performed using 1 3 1 I-labelled seawater. Throughout the experimental phase the winkles were fed on unlabelled Chondrus crispus. 1 3 1 I concentrations in winkles during uptake followed linear first-order kinetics with an uptake half-time of 11 days, whereas for depuration a triphasic sequence with biological half-lives of 4, 23 and 56 days was determined. In general, iodine turnover in winkles via labelled seawater appears to be slower than observed for other molluscs (2-3 days).Most of the activity prior to and after depuration is found to be in the shell, with indications that shell and soft parts accumulate and depurate 1 3 1 I at a similar rate. The operculum displays the highest specific activity of all fractions with a concentration factor of 750 l kg - 1 . Concentration factors for whole winkle, shell, soft parts and digestive gland are in the order of 40-60 l kg - 1 , higher than the IAEA recommended CF value for iodine in molluscs of 10 l kg - 1 . The 1 3 1 I CF in winkles is closer to that of the conservative radionuclides 9 9 Tc and 1 3 7 Cs than the CF of the particle reactive radionuclides 2 3 9 , 2 4 0 Pu and 2 4 1 Am.