The ecological affinities of Aira caryophylleat, A. praecox, Cerastium atrovirens and Vulpia membranacea growing in the dune system at Aberffraw, Anglesey, have been clearly indicated by the investigations discussed in previous papers (Pemadasa, GreigSmith & Lovell 1974; Pemadasa & Lovell 1974a,b). Not only is the timing of the lifecycle of all the four annuals generally synchronous, but also the overall distributional pattern of different species within the dune system and their response to various environmental factors such as soil moisture and inorganic nutrients are remarkably similar. Moreover, two of the species, Aira caryophyllea and A. praecox, are closely related systematically. The available evidence indicates that the majority of the seeds of these annuals are deposited in the immediate vicinity of the parent plants (Pemadasa & Lovell 1974a), and that most of them germinate more or less simultaneously (Pemadasa 1973). Clearly, individuals with such similar ecological requirements are likely to compete for the limited available resources. In the drier areas of the dune system, where most of these annuals are common, the dominant perennial species is Festuca rubra. Pattern analysis showed that the annual species examined are negatively correlated with F. rubra, and it was tentatively suggested that some form of interference is operating to exclude them from the places where the representation of F. rubra is high (Pemadasa et al. 1974). The low vigour and sparseness of dune vegetation have been attributed to low nutrient status of the sand (Pemadasa & Lovell 1974b; Willis 1963, 1965; Willis & Yemm 1961). The sparseness of perennial plants is thought to favour the continued persistence of the annuals in dune habitats (Willis 1963). If this is the case, perennials should be able to eliminate the annuals when the nutrient deficiencies are made good. Unfortunately, little published information is available regarding the effect of soil fertility on the interference between species. In the present investigation two experiments were performed, one to examine specifically the reaction of different annuals to their own density and to compare this with their reaction to the density of associated annual species, and the other to determine the influence of F. rubra on the performance of the annuals. A further aim of the experiments was to study the effect of contrasting nutrient regimes on the interference between species.