The middle Eocene interval at some Paris Basin localities was studied through high-resolution stratigraphy. Abundance data (499 species, 37 719 individuals) on the distribution of molluscs, collected at 12 shell beds of the middle Lutetian and lower Bartonian, formed the basis for a palaeoecological study. The middle Lutetian succession is subdivided into several elementary depositional sequences (EDS) interpreted as the product of relative sea-level change. Species-abundance distributions are better correlated with EDS than with geographical locality, suggesting that sea level played an important role in the distribution of palaeocommunities. Diversities were compared with analogous data from modern subtropical and warm-temperate intertidal and subtidal communities. We found that sea-level variation is responsible for a major change in the upper part of the middle Lutetian succession, leading from high- to low-diversity palaeocommunities. From base to top sampled palaeocommunities indicate a transition from high-energy and mesotrophic (EDS 2) to oligotrophic low-energy conditions of a sandy lower shoreface (EDS 4) to an upper shoreface (EDS 5 and lower Bartonian), the last with mangroves and a seagrass cover. Notwithstanding the Lutetian cooling, we found that subtropical conditions reached as far north as the Paris Basin. Our study suggests that climatic fluctuations might be obscured by facies control.