Estuarine waters are continuously loaded with chemicals which affect the physiology of aquatic organisms to various extents. They also have adverse effects on a wide range of behaviors. Nonylphenols and related compounds are biodegradation products of the nonionic surfactants nonylphenol polyethoxylates. They are commonly found in the aquatic environment. We observed immediate alterations of the free swimming activity of the calanoid copepod Eurytemora affinis from the Seine estuary in response to a sub-lethal concentration of nonylphenols (4-NP and NP1EC) at environmentally realistic concentrations (2 μg/L). Swimming speed and activity increased for both males and females. The use of copepod kinematic proved to be a sensitive indicator of sub-lethal exposure to pollutants.