Spatio–temporal changes in density, breeding cycle, growth and population structure of Hydrobia ulvae (Gastropoda: Prosobranchia) were studied by monthly sampling of a wide bare mudflat in Marennes-Oléron Bay. Four stations, located along a cross-shore transect and characterized by different geomorphological structures, were investigated from March 2000 to February 2001. Hydrobia ulvae was mainly distributed in the upper half part of the mudflat and no individuals were found in the lower part. The breeding cycle extended from March to December and showed two annual peaks, in spring and in autumn. Three cohorts were recruited during the year and showed high growth rates during summer; the parameters of the von Bertalanffy model describing the growth curves were equal to kmax=0·47±0·5 mm month−1 and L∞=5·4±0·2 mm. The snail population had similar size–frequency structure along the transect at the beginning of the survey but summer recruitment initiated spatial differentiation. Reproduction occurred in the middle part of the mudflat but recruits mainly settled down at the upper level of the mudflat; new cohorts appeared with increasing individual densities. The middle part of the mudflat was rather dominated by adult individuals which showed large density fluctuations.