Heterosigma carterae is a noxious alga that blooms globally in temperate and subtropical regions. This study demonstrates that Heterosigma produces a haemolytic toxin. The toxin is sequestered in the cell and can be released by subjecting the alga to freeze/thaw conditions. Heterosigma cell extracts also display ichthyotoxic activity and can impact normal physiogical response in Artemia.   Correlative field studies measured the effects of a broad spectrum of environmental cues (e.g. nutrient load, pH, dissolved oxygen, etc.) before and during a Heterosigma bloom that occurred in 1997.  Data suggest, at least for this noxious event, that shallow salinity and temperature stratification might be triggers that induce rapid cell growth.  The primary goal of this project was to analyze the impact of specific physiological cues on Heterosigma growth, and to isolate and analyze the toxin(s) produced by Heterosigma carterae. The secondary goal of this research was to monitor the effect of Heterosigma toxin on vertebrates and invertebrates. The data presented in the text, demonstrates for the first time, that Heterosigma produces a haemolytic toxin. The haemolysis of sheep red blood cells with saponin as a reference control was used as an assay. It was shown that this Heterosigma toxin is sequestered and that the toxin does not loose activity in frozen cells. Although a large effort was expanded, no correlation between physiological cues and cellular haemolytic toxin level was determined in these studies.  Collaborative studies with W. Gerwick’s laboratory identified a Heterosigma cellular fraction caused severe diarrhea in brine shrimp. Heterosigma cellular fractions were also analyzed through the National Cancer Institute Therapeutic program.