We designed and synthesized fluorescent probes for the sensitive and selective detection of highly reactive oxygen species (hROS) in mitochondria of living cells and confirmed their usefulness in vitro. MitoAR and MitoHR are highly selective for hROS (center dot OH, ONOO-, OCl-) over other ROS. They have exellent properties, including tolerance to autoxidation and photobleaching by laser irradiation during fluorescence microscopy and should be useful tools for biological and pathological applications.