Biomonitoring of thallium availability in two estuaries of southwest England

► Thallium has been measured in sediments and biomonitors of two estuaries. ► Concentrations are higher in sediments than in macroalgae and invertebrates. ► Bioaccumulation of Tl is lower than that of other metals. ► Because of its toxicity, closer monitoring of Tl in estuaries is recommended.