The present invention provides a novel genetically encoded fluorescent indicator of hydrogen peroxide. The indicator comprises a sensor polypeptide which is responsive to hydrogen peroxide and a circularly permuted fluorescent protein which is operatively inserted into the sensor polypeptide. Interaction of the sensor polypeptide with hydrogen peroxide results in a change in fluorescence of the fluorescent protein. An isolated nucleic acid molecule encoding a hydrogen peroxide fluorescent indicator of the present invention is also provided. Also provided are vectors, expression cassettes, host-cells, stable cell lines, transgenic animals and transgenic plants comprising a nucleic acid of the present invention. Also provided are methods that use a fluorescent protein of the present invention or the nucleic acid encoding it.