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It is generally accepted that the bulk of the biologically fixed nitrogen on the surface of the Earth is leguminous in origin. There is good evidence, however, that other nitrogen-fixing organisms, such as the nodulated non-leguminous angiosperms, for example Alnus, Myrica and Casuarina, and free-living nitrogen-fixing micro-organisms, such as the blue-green algae and the bacteria, contribute considerable quantities of fixed nitrogen to the soil ca. The latter groups are probably more important in natural ecosystems than in the cultivated soils of which the legumes are so typical.A natural population which has been studied, particularly from the point of view of nitrogen-fixation by blue-green algae, is that associated with salt marshes 24 and areas of sand dune slack 22. Evidence to date indicates that in these habitats nitrogen-fixation by free-living organisms may be appreciable. At one of the sites studied -Gibraltar Point Nature Reserve, Lincolnshire (British National Grid Reference TF 555576), a large area of the sand dune system is characterised by the abundance of the nodulated non-leguminous angiosperm Hipp@ha~ rhamnoides L., a species which in the laboratory has been shown conclusively to fix nitrogen, both on the basis of long-term growth analyses 6 and by use of the sensitive N la technique. 3 The biology of this plant has recently been reviewed by P e a r s o n and R o g e r s is Because of the abundance and vigour of Hippopha~ at Gibraltar point and thus its large potential contribution to the nitrogen status of the habitat, the experiments to be described in this paper were carried out. These involved measuring the degree of nodulation and --3 4 8 --