An oxidative burst in free-living and symbiotic dinoflagellates induced by physical stress is defined and characterized. The oxidative burst occurred within 1 min of physical injury caused by short pulses of low frequency sonic sound (20 kHz, 10 s pulses). The quantities of reactive oxygen species were measured using a spectrofluorometric assay and standardized to hydrogen peroxide. Using pharmacological probes, the oxidative burst was found to contain upwards of 95% hydrogen peroxide and was believed to be of enzymatic origin. Symbiotic dinoflagellates of the genus Symbiodinium sp. isolated from the gorgonian coral Pseudopterogorgia elisabethae produced a burst that was less than 33% of the magnitude of the oxidative burst in Symbiodinium sp. isolated from Pseudopterogorgia americana and less than 2% of the magnitude of the oxidative burst in the related free-living dinoflagellate, Heterocapsa pygmaea. We recently reported that Symbiodinium sp. from Pseudopterogorgia elisabethae contain high levels of the unique diterpene metabolites, the pseudopterosins, not found in the free-living Heterocapsa pygmaea and Symbiodinium sp. isolated from Pseudopterogorgia americana. Pseudopterosins completely blocked the inducible oxidative burst when applied exogenously to these two dinoflagellate species. Preliminary evidence is presented that raises the possibility that the mechanism could involve inhibition of G-protein activation among many possibilities. These data are the first description of an inducible oxidative burst in dinoflagellates. The significance of the comparative oxidative burst in free-living and symbiotic dinoflagellates is discussed and the potential natural role of the pseudopterosins is postulated.