The mysterious process by which fireflies can control their flashing has inspired over a century of careful observation but has remained elusive. Many studies have implicated oxygen as the controlling element in the photochemical reaction, and the discovery of nitric oxide synthetase (NOS) in the lantern has suggested that nitric oxide (NO) may control oxygen access to the light-emitting photocytes, thereby triggering the flash. However, there are several drawbacks to oxygen as a controlling agent, and in view of the prominence of peroxisomes in lantern morphology and biochemistry, we suggest that it is hydrogen peroxide that triggers the flash, and we present a model by which this may take place.