In Britain apparently similar freshwater localities may contain the isopods Asellus aquaticus and A. meridianus, or only one or other of these species, or Asellus may be absent. It has been suggested, on zoogeographical evidence, that this pattern of distribution is due to interspecific competition. Further evidence in support of this suggestion is put forward in the present paper. Thus a detailed survey of suitable localities on Mersoyside, an investi|gation of the change in the ratio of the two species with time in two localities (Llyn Llywenan, Lake Windermere), and a transference experiment all support the suggestion that A. meridianus and A. aquaticus are in competition, and that the former species is gradually being replaced by the latter. The data from a number of laboratory experiments proved largely inconclusive.