The factors controlling the precise time of germination of seven species of winter annuals, Aira caryophyllea, A. praecox, Cerastium atrovirens, Erophila verna, Mibora minima, Saxifraga tridactylites and Vulpia membranacea from sand dunes at Aberffraw have been investigated. By comparison of germination in natural conditions with that in controlled laboratory conditions, it is shown that the germination of these species is normally controlled by an interaction of dormancy, soil moisture, temperature and the changing temperature response of the seed with ageing. The adaptive significance of continuous and simultaneous types of germination in relation to these annuals is briefly discussed.