We investigated different compounds for activity in mitigating the lethal effects on fish of two dinoflagellates, Gymnodinium mikimotoi and the morphologically similar G. cf. maguelonnense. Measurements were made of survival of seabass in cultures of both species of dinoflagellate, flow of the cultures through the channels between their gills, and on haemolytic activity of the cultures. Both dinoflagellates showed a strong tendency to kill fish. G. mikimotoi at 23,000 cells/ml reduced the survival time of fish 40%, but showed no detectable haemolytic activity. G. cf. maguelonnense at 3000 cells/ml, however, reduced survival time by 74% and showed high haemolytic activity. G. mikimotoi showed more rheological acitivity, while G. cf. maguelonnense was the more haemolytic. Two compounds were found to mitigate the rheological and haemolytic activities of these two dinoflagellates, as well as their tendency to kill fish. These compounds were N-acetyl-L-cysteine and ethyl-L-cysteine ester. The optimal concentration of both was around 0.1 mM (~155 g/m³). Each compound therefore reduced the fish-killing tendencies of both dinoflagellates. By treating water containing fish stocked or concentrated at suitable density (~4 kg/m³) in a fish farm, the cost per treatment would be 0.3 to 3 US cents/tonne of fish.