The Irish Sea has strong tidal oscillations of heights up to 9 m at springs, and is the meeting point of cold-temperate and lusitanian faunas. Well defined tidal fronts exist at the northern thip (Malin Head) and on the east coast around Strangford Lough. The distribution of the fauna and flora of the intertidal zone at these fronts show clear discontinuities, due in part at least to the degree of mixing through tidal energy. The contribution of other factors, notably that of estuarine warming in summer, of dispersive influences at headlands, and of barren stretches of sandy coasts, subject to severe scour, all contribute to the quantitative biogeographical distribution of northern and southern elements of the Western European marine flora and fauna. In conclusion, the influence of tidal fronts and of other factors is discussed in relation to biogeographical limits of intertidal species outside Europe. The paper emphasises that persistent frontal systems have influences extending beyond the pelagic environment.