A commonly accepted paradigm in the study of saxitoxin-producing dinoflagellates is that the total concentration of all toxins (toxin content) in one isolate can vary with growth conditions, but that the relative abundance of each toxin (toxin composition) does not change. We demonstrate here that dramatic changes in toxin composition do occur in one isolate of Alexandrium fundyense. In nitrogen- and phosphorus-limited semi-continuous cultures, toxin composition varied systematically with growth rate. When cells grew slowly under severe nutrient limitation, toxin composition was dominated by one or at most two toxin epimer pairs; as nutrient stresses eased at higher growth rates, the toxin profiles became more heterogeneous. Steady-state, sustained nitrogen limitation favored the production of toxins C 1,2 and GTX I,IV, whereas phosphorus limitation produced cells with high relative abundance of GTX II,III. STX reached its highest relative abundance when growth was most rapid. The lack of observed compositional changes in most past studies is probably not due to inherent differences in toxin biosynthetic pathways between the strains of Alexandrium examined, but rather to differences in the physiology of cells grown under different culturing modes (batch vs semi-continuous), methods of toxin analysis, and dominant toxins in the particular isolates examined.