A new boron-dipyrromethene (BODIPY) derivative (FS1) containing two triazole units exhibits an enhanced fluorescence in the presence of Hg 2+ ions and a high selectivity for Hg 2+ ions over competing metal ions in methanol: Ag +, Ca 2+, Cd 2+, Co 2+, Cu 2+, Fe 2+, Fe 3+, K +, Mg 2+, Mn 2+, Ni 2+, Pb 2+, and Zn 2+ produced only minor changes in the fluorescence of FS1. The apparent dissociation constant (K d) of FS1-Hg 2+ was found to be 62 μM. Moreover, fluorescence microscopy experiments showed that FS1 can be used as a fluorescent probe for detecting Hg 2+ ions in living cells. A new BODIPY-based fluorescence chemosensor FS1 shows high selectivity for Hg 2+ ions over other transition-metal ions. The fluorescence of FS1 was significantly enhanced in the presence of Hg 2+, with green emission, and the addition of other metal ions barely affected the fluorescence. This FS1 chemosensor is an effectivemethod for Hg 2+ sensing in living cellimaging.