The Bell Beaker complex is defined, above all, by a ceramic style widespread across Europe during the 3rd millennium BC. Its particularly large geographic distribution has provoked different interpretations: a unique population invading Europe, the long-distance exchange of prestige goods, and the absence of a real Bell Beaker population with only the diffusion of its cultural components. For Western Switzerland, the Bell Beaker period would have developed following influences varying from both the Mediterranean region and Central Europe (Gallay 2006). Bioanthropology makes it possible to test the first of these hypotheses, which proposes the diffusion of a culture by population displacement. Our previous research on dental nonmetrics supports the idea, for Western Switzerland, of a certain harmony in Middle Neolithic populations and mobility or a moderate population contribution beginning in the Final Neolithic and continuing more intensely during the Bell Beaker period (Desideri 2007). In order to validate and quantify this external population contribution during the Bell Beaker period in this region, enamel of 23 Bell Beaker individuals from the necropolis of the Petit-Chasseur (Sion, Valais, Switzerland) have been analyzed for strontium isotope ratio (87Sr/86Sr).