In this research, biodegradation associated with evaluation of the variations of acute toxicity of 5 anionic detergents towards Anguilla anguilla is described. It appears that the process of anionic detergent biodegradation in sea water is similar to the results obtained by many authors working on fresh water. The experimentation shows 2 different kinds of behavior among the 5 anionic detergents tested: (a) the first category is composed of syndets, without aryl group in their formulation, which are promptly oxidized by bacteria and do not reveal variation of acute toxicity during their biodegradation; (b) the other group includes LAS; those syndets need a quite long bacterial acclimation. On the other hand, their toxicity is reduced whilst they are degraded.