Anion recognition and anion sensing are of interest because anions play many important roles in living organisms. Most currently known anion sensors work only in organic solution, but sensors for biological applications are required to function in neutral aqueous solution. We have designed and synthesized a novel fluorescent sensor for anions. The sensor molecule 1-CdII contains 7-amino-4-trifluoromethylcoumarin as a fluorescent reporter and CdII-cyclen (1,4,7,10-tetraazacyclododecane) as an anion host. In neutral aqueous solution, CdIIof 1-CdIIis coordinated by the four nitrogen atoms of cyclen and the aromatic amino group of coumarin. When various anions are added to 100 mM HEPES buffer solution (pH 7.4) containing 1-CdII, the aromatic amino group of coumarin is displaced from CdII, causing a change of the excitation spectrum. While pyrophosphate and citrate were detected with high sensitivity, fluoride and perchlorate produced no response. Among organic anions, ATP and ADP gave strong signals, while cAMP showed little signal. By utilizing the different affinities of the sensor for AMP and cAMP, the activity of phosphodiesterase, which cleaves cyclic nucleotide, was monitored in real-time. The sensor should have many biochemical and analytical applications and the sensing principle should be widely applicable to the sensing of other molecules.