The raphidophyte Chattonella marina, is a known ichthyotoxic species associated with fin-fish mortalities in aquaculture. The loss of $A45M of farmed bluefin tuna in South Australia in 1996 raised the question of differences in toxicity between the Australian and Japanese strains of C. marina. Autecological studies demonstrated that the Japanese and Australian strains had similar tolerances for temperature and salinity, but substantial differences in irradiance requirements, with the Australian strain showing accelerated growth under irradiances of 400 µmol m-2 s-1 (Marshall & Hallegraeff. 1999. J. Plank. Res. 21, 1809-1822). This adaptation to high light intensity related to the presence of the Mycosporine- like amino acids (MAAs), shinorine, mycosporine-glycine, and mycosporine-glycine:valine, at levels 6 to 8 times higher in the Australian strain than the Japanese strain, with mycosporine-glycine absent in the Japanese strain. Mycosporine-glycine, a noted antioxidant, may reduce the effect of the reactive oxygen species (ROS) in the Australian strain. The effect of irradiance on ROS production was determined for both the Australian and Japanese strains using chemiluminescence, zooplankton Artemia salina and fish Onchorynchus mykiss as bioassays. The relative importance of ROS production and neurotoxins as fish killing mechanisms by this alga are discussed.