INTRODUCTION Regulation of luciferase (LCF) activity in dinoflagellates is unique, without precedent in other enzymes. In brief, it involves control of substrate binding. At pH 8 its conformation prevents substrate from binding. At pH 6 the LCF conformation changes, allowing the tetrapyrrole luciferin (LH2) to access the binding site. The reaction then occurs giving a bright, brief flash of less than 100 msec. In at least one species, (Lingulodinium polyedrum, formerly Gonyaulax polyedra), regulation is enhanced by the action of a second protein, luciferin binding protein (LBP), which binds luciferin at pH 8 but releases it at pH 6.1 Such a large and rapid pH jump is possible by virtue of the fact that the light emitting system is contained in small (~0.5 µm), cortically located novel organelles named scintillons, numbering about 400 per cell. They occur as outpocketings of the cytoplasm, projecting into the cell vacuole, connected like a balloon, and containing only LCF, LH2 and LBP. They can be identified by immunolabeling with antibodies raised against LCF or LBP, and visualized by their bioluminescent flashing following stimulation, which is co-localized with the fluorescence of luciferin. Based on the effects of pH on the activities of purified LCF and LBP, and also on isolated scintillons, in vivo flashing was postulated to result from a transient pH change in the scintillons, triggered by a mechanically initiated action potential in the vacuolar membrane which opens ion channels that allow protons from the acidic vacuole to enter. As they are effectively isolated from the parent cytoplasm, very few protons are needed to change the pH from 8 to 6.