We describe three significant advances in the use of thioureas as reporting elements for metal-responsive fluorescent chemosensors. First, on the basis of the crystal structure of a chemosensor analogue, we provide a deeper understanding of the details of the thiourea coordination environment. Second, we describe a new generation of chemosensors with higher affinities for Zn(2+) and Cd(2+) than were observed for earlier probes, expanding the scope of this type of probe beyond Hg(2+) detection. Third, we show that a thiourea-based chemosensor can be employed for fluorescence microscopy imaging of Hg(2+) ion concentrations in living mammalian cells.