In this study, we investigated the production of hydrogen peroxide (H2O2) by the red-tide organism Chattonella marina. Subsequently, we examined the antioxidant responses as well as lipid peroxidation in gills and hepatopancreas of the mussel Perna viridis upon exposure to C. marina at environmentally realistic concentrations (103 and 104 cells ml 1). Despite the extracellular levels of H2O2 generated were up to 0.5 (at 103 cells ml 1) and 20 mM (at 104 cells ml 1), no significant differences could be observed in any of the examined biochemical parameters (i.e. catalase (CAT), glutathione (GSH), superoxide dismutase (SOD), glutathione peroxidase (GPx), glutathione-S-transferase (GST), glutathione reductase (GR) and lipid peroxidation (LPO)) in mussels between treatment and control groups. In another experiment, where mussels were exposed to H2O2, no responses were induced at the concentration up to 0.5 mM. These data collectively indicate that the production of H2O2 by C. marina is not high enough to elicit antioxidant responses in mussels. As such, reactive oxygen species (ROS) is unlikely to be an important toxicological mechanism of C. marina.