Hypochlorous (HOCl) acid is generated as a defense tool in the immune system and plays a vital role in killing a wide range of pathogens. There is therefore great interest in developing fluorescent probes that can endogenously respond to the change in concentration of HOCl in vivo. To address this challenge, we here present a rapidly responding fluorescent probe RO610 to image endogenous HOCl in living mice. The development of RO610 was based on a novel water-soluble and pH-independent fluorescent xanthene dye, 2'-formylrhodol ROA, which exhibits highly selective and sensitive responses to HOCl/ClO- over other reactive species. Moreover, adding a little more than 5 equiv. of ClO- to the solution of RO610 resulted in a clearly observable fluorescence enhancement (48-fold) within 30 s. Based on these properties, RO610 was used to detect ClO- in A549 cells without interference by other oxidants. It was applied for the imaging of endogenous HOCl in living nude mice with satisfactory results.