Previous papers (Pemadasa, Greig-Smith & Lovell 1974; Pemadasa & Lovell 1974) provide a quantitative description and a causal interpretation of the distribution of a group of annuals in the dune system at Aberffraw, Anglesey. Most of these species are characterized by autumn germination and spring flowering, and are, therefore, regarded as 'winter annuals'. Extensive information is available regarding the timing of the life-cycle of winter annuals of the Derbyshire limestone (Ratcliffe 1961), of the East Anglian Breckland (Newman 1963) and of the cedar glades of Tennessee (Baskin & Baskin 1971a,b, 1972). A series of investigations has been undertaken to explore the factors controlling the timing of the life-cycle of winter annuals of the dune system at Aberffraw, and the present paper is concerned with the factors determining the precise time of flowering in the field of Aira caryophyllea, A. praecox, Cerastium atrovirens, Mibora minima and Vulpia membranacea. Nomenclature follows Clapham, Tutin & Warburg (1962). Although all these species generally germinate in autumn (Pemadasa 1973), flowering does not occur until the following spring. Their flowering may be delayed until the spring because (a) the short time available for growth prevents them assuming the 'competenceto-flower' in autumn, (b) the winter temperatures are so low that they hinder flowering, (c) there are highly specialized environmental requirements for flowering which are not satisfied until the spring, or (d) interaction between any or all of these factors. These possibilities were examined experimentally.