“…They readily accumulate many metals and their body burden seems to reflect mean exposure levels over time (Naimo, 1995). Consequently, such organisms have been largely used in programmes of biological monitoring in either salt water (Farrington, 1983;De Gregori et al, 1994;McConnell and Harrel, 1995;Avelar et al, 2000) or in freshwater (Manly and George, 1977;Foster and Bates, 1978;Millington and Walker, 1983; ) Abaychi and Mustafa, 1988;Hameed and Raj, 1990;Kraak et al, 1991;Camusso et al, 1994;Valdovinos et al, 1998;Villar et al, 1999;Rutzke et al, 2000). The species adopted as a biological monitor in this study (A. trapesialis) has been successfully used by different investigators (Avelar et al, 1991;Roma and Longo, 1991;Lopes et al, 1992) in monitoring programmes on some rivers in São Paulo State.…”