Plant size and aspects of the reproductive potential of monotypic and polytypic populations of Fucus spiralis and Fucus spiralis forma nanus were compared. The two formae were distinct, but the differences between them were less pronounced where they co-occurred, suggesting the possibility of a level of inter-forma gene flow. Fucus spiralis f. nanus plants pay a potential reproductive cost in that they produce smaller eggs than F. spiralis, but the two formae appear to have similar levels of total reproductive output (eggs per unit of receptacle tissue).