Sediments act as sinks for contaminants of natural and anthropogenic origin, constituting a risk to the living organisms. In this study, sediments were collected from three sites on the coast of southwest France. The objective of this research was to determine the effects of sediments on embryonic development of bivalves and to identify precisely when the contaminants affect the embryos and induce them to develop in an abnormal way. The toxicity of decanted sediments and overlying waters were assessed using the oyster embryo bioassay. The physical characteristics and contaminant levels in the sediments were measured, including polycyclic aromatic hydrocarbon (PAH) and metal concentrations. Despite contaminant concentrations for PAH and metals only exceeding the effects range-low levels, all decanted sediments tested induced deleterious effects on the embryonic development of oysters, while no significant abnormalities were observed for overlying waters. The study results suggest that abnormal larvae mainly are caused by direct contact with contaminated sediments.