On rocky shores of the British Isles and neighbouring regions of the North Atlantic coastline, the sporophyte of Laminaria hyperborea (Gunn) Foslie is the dominant member of most sublittoral algal communities. The bathymetric distribution, like the geographical distribution, is extensive; L. hyperborea is often dominant from extreme low water level of spring tides (E.L.W.S.) to depths of up to 32 m. The great changes in environment encountered by this species over this wide range of distribution would suggest a highly adaptable organism. Changes in size, rate of growth and growth pattern have indeed been recorded between plants at different latitudes and different depths. Plants from northerly waters generally are much larger with longer stipes and have a greater frond area (Kain, 1962; Bellamy & Whittick, 1968; Greenhager, 1958), although Kain (1963, 1967) points out that this is often due to greater longevity of individuals rather than greater growth rates. Increasing depth affects the density of distribution; the typical L. hyperborea forest becomes more sparse until an open community, called 'the park' when first recorded by Kitching in 1941, replaces the forest at about 15 m. The plants in the open community are smaller than those in the forest and have shorter stipes (Kain, 1962, 1966, and observations described in this communication). However, despite these changes in growth,, almost no morphological variation had been recorded prior to the present investigation. More recently there has been a report of a form L. hyperborea f. cucullata (Svendsen & Kain) from deep water and waters not subject to water movement with thin enlarged fronds and short stipes (Svendsen & Kain, 1971; Kain, 1971).