The effect of fluctuations in temperature and moisture supply on hard seeds of nine tropical coastal sand dune legumes, including herbs (Schrankia atiadrivalvis, Macropiilium atropurpureum and Canavalia rosea), and shrubs (Acacia farncsiana, A. macracantha, Mimosa chaetocarpa, Indigoftra sujjruticosa, Crolalaria incana and Chamaecrista chamaecrutoides), has been studied under laboratory and field conditions. Using a fluctuating temperature gradient bar seeds buried in sand were exposed to various amplitudes of diurnal temperature fluctuation over an extended period of time and seed germinability was examined at intervals. Germination percentage increased considerably in most species as a consequence of treatment with marked effects occurring at temperature fluctuations greater than 20°C and becoming detectable after 45 days. In several species the effect of applying wetting and drying cycles was to lower the amplitude of temperature fluctuation necessary to soften the seeds. Species can be grouped into physiological groups on the basis of differences in seed response to temperature fluctuations. It is suggested that these differences may help to explain the mechanisms distinguishing the different colonization patterns observed in tropical sand dune systems.