The unicellular marine Raphidophyte, Heterosigma sp. is associated with coastal fish kills in Japan, New Zealand and Canada. This phytoplankter is common in many coastal waters but is reported to be toxic in specific geographical areas. The mechanism of toxicity is poorly understood and could be due to either the production of a compound similar to brevetoxin or the abnormally high production rates of reactive oxygen species (ROS). One such ROS is hydrogen peroxide (H2O2). Production of H2O2 is not a constitutive property. The levels of production vary depending on the origin of the isolate, the phase of growth and the environmental conditions for growth. Several of our isolates show rates of production sufficient to achieve an extracellular concentration as high as 5 x 10-7 M. In order to determine if these levels of ROS are sufficient to negatively influence fish, we have perform a series of experiments on isolated cell lines. For comparison, we have examined the sensitivity of a fish gill cell line with the more traditional mouse liver cell line. We have considered if the Heterosigma cells alone or the spent medium from the different cultures grown under different environmental conditions can damage the cell line and if this damage is identical to the destruction due to direct exposure to H2O2. Collectively, this data could determine the feasibility of algal-produced H2O2 as an ichthyotoxic agent during fish kills.