In the present work chitosan (CT) was used to construct a chitosan-modified carbon paste electrode (CT/CPE). The amine groups in the chitosan bind mercury ions from an aqueous KCl solution (PH 5.0). Then the electrode was held at a negative potential (-1.3 V) to reduce the Hg2+ bound to the CT/CPE to forming mercury nanodroplets. The proposed electrode was evaluated for preconcentration and determination of Tl+1 ions by differential pulse anodic stripping voltammetry (DPASV). The electrochemical procedure was based on the accumulation of thallium ions at the surface of the modified carbon paste electrode with mercury nanodroplets and chitosan biopolymer. Parameters involved in the binding of Hg+2 ions to the CT/MCPE, preconcentration of thallium ions and voltammetric stripping step were studied and the best set of experimental conditions were chosen. Under the optimized conditions, the voltammetric peak current of Tl+1 increased linearly with Tl+1 concentrations in the range of 1.0-400 ng mL(-1) and a detection limit of 0.20 ng mL(-1) was obtained for Tl+1. The practical application of the suggested modified electrode was evaluated by the determination of trace amount of Tl in the environmental and biological samples. (C) 2017 The Electrochemical Society. All rights reserved.