The correlation between changes in length of the different cell cycle stages and the toxicity of Alexandrium fundyense Balech was studied in semi-continuous cultures. Growth rates ranging from 0.031 d(-1) to 0.36 d(-1) were established at different temperatures or levels of phosphate limitation. In all treatments, G1 was the phase with the longest duration. Decrease in growth rate was associated with an increase in duration of the different cell cycle stages. Toxin content was always directly correlated to the duration of the G1 phase. In both the temperature treatments and the phosphate limitation experiments, toxin production rates remained constant for the respective range of conditions, implying that the variations in toxin content observed were a result of increasing periods of biosynthetic activity. Toxin accumulation was directly correlated to protein biosynthesis in all temperature treatments. In contrast, toxin content showed little correlation with protein content as phosphate limitation increased. Significant differences in toxin composition were observed between the temperature and phosphate treatments. Total concentrations of GTX II and III and C I and II were significantly higher in the phosphate-limited cultures, while the levels of STX, NEO and gonyautoxins I and IV remained virtually unchanged. We conclude that toxin biosynthesis in A. fundyense is coupled to the G1 phase of the cell cycle, that toxin synthesis is not down-regulated by phosphate deprivation and that interconversions among saxitoxin derivatives are influenced by the availability of phosphate.