A novel conjugated polymer (RB-PPETE) of poly [p-(phenylene ethynylene)-alt-(thienylene ethynylene)] (PPETE) bearing covalently linked thienylene rings and Rhodamine B units has been synthesized and successfully used to detect metal ions. The Rhodamine B exists as a lactone, which is colorless and non-fluores cent. Hg2+ ions can induce the Rhodamine group to form a ring-opened state. The fluorescence resonance energy transfer (FRET) was demonstrated in the polymer, and in the presence of Hg2+ ions the excitation energy along the backbone of the conjugated polymer is transferred to the energy acceptor (Rhodamine B), which leads to a visual color change of the solution from slight yellow to orange. Meanwhile, this new system shows outstanding Hg2+-selective FRET off-on type fluoroionophoric properties among the representative metal ions in tetrahydrofuran.