Two brief fishkill episodes occurred in the Murray river, Western Australia, on June 2 and June 8 1999. Physical profiles of the water column were taken to check for deoxygenation, while integrated and grab phytoplankton samples were also collected for analysis. With only low levels of H2S and NH3, and satisfactory O2- concentrations, none of the physical attributes of the water column could be linked to the fishkill. Pathological tests on fresh dying fish collected from the site concluded that liver and gill damage symptoms were consistent with effects of toxic dinoflagellate algae. Substantial concentrations of a small dinoflagellate (12-17mm) were detected close to the reported fishkill site on June 2 and again on June 8. With normal light microscopy techniques this dinoflagellate was tentatively identified as Gymnodinium cf. galatheanum Braarud. The identity of the dinoflagellate in question was further investigated with techniques including Nomarski (DIC) phase microscopy, fluorescence microscopy and electron microscopy. Evidence gathered suggests that the dinoflagellate species observed in the Murray River at the time of the fishkills was Gymnodinium galatheanum Braarud, a species previously reported to be icthyotoxic (Abboud-Abi Saab, M. & Y. El-Bakht, 1998; Johnsen, G. et al., 1998).