The effects of supplementation with bicarbonate and K-nutrients on the growth and C2 toxin (C2T) productivity by a model dinoflagellate, Alexandrium tamarense ATCI01, were investigated in batch cultures. Supplementation with bicarbonate at a level of 5mM increased both the cell density and toxin yield to 140 and 210%, respectively, compared with the non-supplemented control culture, and achieving 13.6x10 3 cells/ml and 875μgtoxin/l. Supplementation at the start or at day 6 of the cultures produced similar effects. The results confirm that carbon can be limiting in cultures of this photosynthetic dinoflagellate, where dense growth was intentionally maintained for high toxin yield. When the cultures were supplemented both with bicarbonate at 5mM and K-nutrients to double the nitrate and phosphate supply, the cell density and toxin yield further increased to 29.8x10 3 cells/ml and 1080μgtoxin/l, respectively, regardless of whether the supplementation of the cultures was made at the start or at day 6. The results indicate that supplementation with bicarbonate and K-nutrients would increase the yield of C2T in mass production operations using this model dinoflagellate strain.