A new rhodamine-hydroxamic acid-based fluorescent chemosensor for the rapid detection of HOCl in aqueous media was developed. The system, which utilizes an irreversible HOCl-promoted oxidation reaction, responds instantaneously at room temperature with linear proportionality to the amount of HOCl. This system is highly selective for HOCl over other reactive oxygen species (ROS) and highly sensitive in aqueous solutions. Biological imaging studies using living cells and organisms (A549 cells and zebrafish) to detect HOCl are successfully demonstrated.