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AbstractSeveral biological and ecological characteristics of Galium minutulumJordan, a rare steno—mediterranean annual species, were investigated on some populations of the Hyères islands, which are located in the north—eastern limit of the species distribution. The objectives of this study were to better document distribution, autecology and the form of rarity of this species in Provence and to propose accordingly some elements for a sustainable management of that vulnerable taxa. Galium minutulumexhibits a noteworthy polymorphism and an important ecological plasticity, which were both mostly explained by the existence of two allopatric cytotypes. Tetraploid individuals, small—sized and spindly, are located in the core—areas of the biggest islands (Porquerolles, Port-Cros, Le Levant), at the edges of the sclerophyllous matorrals or the thermophilous holm- oak forest. Hexaploids, more robust, grow on the coastal habitats directly exposed to sea sprays, notably on the smallest islands (Bagaud, Grand Ribaud). Many factors such as meiosis abnormalities, reduction of germination capacity of the tetraploid cytotype, frequent infestations of a pathogene fungus (blight), and the sensitive locations near the tracks and paths, generate several threats on this species and contribute to its rarity.