A seasonal study of the changes in cold-hardiness of Fucus vesiculosus L. was made over a two-year period. Plants in summer could withstand about –30° C. (the lowest temperature at which 50% of the frond was still alive after treatment), whereas in January and February, plants could withstand –45° C. to nearly –60° C. Changes in hardiness with the season appeared to be related to air temperature variations, but also to mean monthly surface water temperatures plotted for a year. In May, Fucus taken from the low tide level was 15° C. more sensitive to the cold treatment than those from the highest level of the fucoid zone. Growing tips were the hardiest part of the plants in winter and early spring, but the least hardy in late summer.