Phenotypic variation of Fucus spiralis was examined in terms of morphology and chemical composition, as determined by pyrolysis mass spectrometry (PyMS). After discriminant analysis both techniques revealed the existence of two morphotypes which were subsequently designated as F. spiralis and F. spiralis forma nanus. The nanus plants differed from the nominate spiralis in that they were shorter, narrower and had fewer branches, but had more units of growth from a single holdfast. F. spiralis f. nanus was consistently higher on the shore than F. spiralis, but the two morphotypes overlapped in terms of shore exposure level. We suggest that the formae exist within a mosaic of stable phenotypes representing populations specifically adapted to the local environment. PyMS data were in agreement with morphological descriptions showing that gross morphology can be correlated with chemical composition.