Uptake and assimilation kinetics of nitrate and ammonium were investigated along with inhibition of nitrate uptake by ammonium in the harmful dinoflagellate Alexandrium minutum Halim at different nitrogen (N)-limited growth rates. Alexandrium minutum had a strong affinity for nitrate and ammonium (Ks50.26 0.03 and 0.31 0.04 lmol .L 1, respectively) whatever the degree of N deficiency of the cells. Ammonium was always the preferred form of nitrogen taken up (rNO3=rNH4 50.42-0.50). In the presence of both forms, nitrate uptake was inhibited by ammonium, and inhibition was particularly marked in N-sufficient cells (Imax~0.9 and Ki50.31-0.56 lmol .L 1). In the case of N assimilation, ammonium was also the preferred form in N-deficient cells (rassim NO3=rassim NH4 50.54-0.72), whereas in N-sufficient cells, both N sources were equally preferred (rassim NO3=rassim NH4 50.90-1.00). The comparison of uptake and assimilation rates highlighted the ability of A. minutum to significantly store in 1 h nitrate and ammonium in amounts sufficient to supply twice the daily N requirements of the slowest-growing N-deficient cells. Nitrogen uptake kinetic parameters of A. minutum and their ecological implications are discussed.