On the rocky shores of the British Isles three species of Fucus are commonly found as components of the intertidal algal belt, namely, F. spiralis L., F. vesiculosus L. and F. serratus L. A fourth species, F. ceranoides L., occurs in places subject to the influence of fresh water but, although it may be important in connexion with the interacting processes to be described below, F. ceranoides is omitted from the present discussion because as yet too little is known of its autecology. Under the conditions prevailing in the characteristically zoned algal flora of an undisturbed region of a rocky coast, descriptions of the three species can be applied which fairly effectively distinguish one from the others. The chief characters used for this purpose are shown in Table I.