The origin and development of a Festuco-Agrostidetum is described from an area of infertile sand. For the most part the species are the same in the different stages of the succession. Polytrichum piliferum is important as pioneer and accumulator of blown sand, lichens are fairly numerous and 2 of them, Cetraria aculeata and Cladonis silvatica, play leading parts, while higher plants are few, the most important being Festuca ovina and Agrostis alba and A. tenuis. The spp. of Agrostis behave in the same way and are considered together. Where there is richness in annual plants in keeping with the poverty of the soil only the least exacting species[long dash]Aira praecox and Teesdalia nudicaulis[long dash]are found. There is no continuous progression up the inclined plane of seral development from the simple to the more complex community with a corresponding increase in soil organic matter, but each stage consists of a series of progressive and retrogressive phases separated by a peak stage. The succession in short is wave-like or cyclic. The early stages occur at the foot of eroding dunes where an erosion pavement checks further removal of sand. Marginal invasion by Polytrichum piliferum leads to its dominance and sand accumulation. There follow stages dominated by Cetraria aculeata. Under the mat of mixed Cladonia-Cetraria, Polytrichum dies and the mat disintegrates in whole or in part, exposing the soil to partial or complete erosion down to the erosion pavement. On the erosion pavement the cycle begins again. Further development of the vegetation is conditioned by the presence of higher plants. Festuca and Agrostis become established vegetatively (Festuca in favorable seasons by seed) in the bare stage from surviving parts brought down from the eroding bank. Their influence is local, that of Festuca being confined to a narrow ring round the tuft, that of Agrostis to the patch of soil it occupies. At this stage of development the crest of the wave consists of patches of Agrostis in a mat of Cladonia, set in a background of Cladonia-Cetraria studded with scattered Festuca. The crest of the next wave is also patchy: patches of abundant Agrostis and relatively few Festuca are set in a background with few Agrostis but many Festuca. Both have an almost continuous carpet of Cladonia. The crest of the late stage is an intimate mixture of abundant Agrostis and numerous but small Festuca again set in a carpet of Cladonia. None of these peak stages is stable; the Agrostis dies in whole or in part and the lichen carpet disrupts, leaving the soil open to erosion. The relationships between Festuca and Agrostis are described for each of the 3 stages. Where Agrostis invades a lichen community with Festuca, Cladonia becomes dominant and the number and size and therefore the total area covered by Festuca decreases. The cover per cent of Festuca in communities with abundant Agrostis tends towards 2.5. There is in each main stage a progression towards this apparent equilibrium which fails of achievement because the community disintegrates before it is reached. While a series of dry years speeds up retrogression relatively to progression both stages exist side by side. Periodicity is inherent in the vegetation itself and connected in some way with the lichen cycle. The primary cause remains obscure.