Population structure and recruitment pattern of undescribed mytilid bivalves that dominate the animal communities associated with vents of the Lucky Strike and Menez Gwen hydrothermal vent fields (37 degrees 17' N and 37 degrees 50' N on the Mid-Atlantic Ridge) were studied from size-frequency analyses. Intra-site comparisons reveal that length-frequency distributions differ significantly from one location to the other. This spatial segregation of the sizes could be due to spatio-temporal variability in environmental conditions and to intraspecific competition processes, especially adult-larval interactions (competition for space and food resources, intraspecific larviphagy) that could prevent larvae from settling on surfaces covered by adults. A modal decomposition analysis was conducted after pooling samples collected from different locations at the same site. Polymodal structure of length-frequency distributions indicates that recruitment is likely discontinuous in time. The presence of postlarvae in each population of the Lucky Strike vent field suggests that a recruitment event took place just before sampling in June 1994, and that mytilid recruitment is synchronized between sites, which is confirmed by the comparison of modal values. Length-frequency distribution of the Menez Gwen mytilid population displays similar modal values, especially for the smallest mode. This suggests a synchronization of the recruitment between the 2 vent fields, which are separated by a distance of 60 km, No seasonal pattern in recruitment could be identified although newly recruited individuals were observed in June 1993 on Lucky Strike sites.