Discriminant function analysis was used to identify two morphotypes of Fucus spiralis (Phaeophyta: Fucales), F. spiralis forma spiralis and F. spiralis forma nanus, within a single population. Evidence was found that they are true formae, not extremes of a range of types produced in response to environmental gradients. Forma spiralis tends to be larger than forma nanus, and to dominate the lower part of the species distribution on the shore. However, size alone appears insufficient to reliably separate the two formae given their high degree of morphological variability.