The possible relationship which exists between size (as weight or length) and mercury concentration in muscle is used by some authors as evidence of the degree of mercury contamination present in aquatic environments. In this study, the level of mercury contamination present in populations of red crayfish (Procambarus clarkii) and grey mullet (Liza ramada) of the Marshes of the River Guadalquivir, both of great interest as food in the Marshes surroundings, has been shown. At the same time, the validity of using the relationship between size (as weight or length) and mercury concentration in muscle, as evidence of the mercury contamination present in the aquatic environment investigated, is discussed.