Three new low-affinity fluorescent Ca 2+ indicators excitable with visible light, namely 3-phenyl-6-(4-(3-carboxymethoxy-4-(N,N-dicarboxymethylamino)phenyl)phenyl)-2,5-dicarboxymethyl-1,4- dihydropyrrolo(3,4-c)pyrrole-1,4-dione (DPP1), 3-phenyl-6-(5-(3-carboxymethoxy-4-(N,N-dicarboxymethyl- amino)phenyl)thien-2-yl)-2,5-dicarboxymethyl-1,4-dihydropyrrolo(3,4-c)pyrrole-1,4-dione (DPP2) and 3- (thien-2-yl)-6-(5-(3-carboxymethoxy-4-(N,N-dicarboxymethylamino)phenyl)thien-2-yl)-2,5-dicarboxymethyl- 1,4-dihydropyrrolo(3,4-c)pyrrole-1,4-dione (DPP3) have been synthesized and evaluated for their Ca 2+ binding properties via fluorimetric titrations. The in vitro dissociation constant Kd measured at 21 ◦ C in 100 mM KCl buffered solution, pH 7.05, for the Ca 2+ -DPP1 complex is 10 µM; for Ca 2+ -DPP2 and Ca 2+ -DPP3 a Kd value of 20 µM is found. All three indicators form 1 : 1 complexes with Ca 2+ . The fluorescence quantum yields of the uncomplexed forms of DPP1, DPP2 and DPP3 are 1.2 × 10 −2 ,3 .4 × 10 −2 and 3.6 × 10 −2 , respectively. After binding to Ca 2+ these values increase to 4.8 × 10 −2 ,5 .0 × 10 −2 and 5.1 × 10−2, respectively.