Fresh weight (FW), dry weight (DW), carbon and nitrogen content were measured for specimens of Laminaria saccharina (Heterokontophyta: Phaeophyceae) sampled in the eastern English Channel in order to conduct a biometrical study. The aim was to relate carbon and nitrogen masses of the algae to a simple and rapid morphological measurement of the total length of the sporophyte. These relationships were highly significant and appeared very useful to express the standing biomass of L. saccharina in terms of carbon or nitrogen and then to consider dynamic processes such as primary production. Variations in tissue carbon (C) and nitrogen (N) were examined over a complete seasonal cycle. Average carbon and nitrogen content ranged from 23·9 to 31·4% and 2·23 to 3·42% of the total dry weight, respectively. Variations in C/N ratio showed a clear seasonal pattern with an increase in the early spring corresponding to strong photosynthesis and growth.