This work is focused on the influence of dilution rate (0.08 <= D <= 0.32 d(-1)) on the kinetics of continuous cultivation of Spirulina platensis at two different concentrations of ammonium chloride (N-0 = 1.0 and 10 mM) as nitrogen source. Cell productivity increased in both series of runs up to D congruent to 0.12-0.16 d(-1), and then decreased. While at N-0 = 1.0 mM biomass washing was certainly the cause of progressive cell concentration decrease, a combination of this phenomenon with the toxic effect of excess ammonia was responsible, at N-0 = 10 mM and D >= 0.20 d(-1), for quick stop of cell growth just beyond the achievement of maximum cell productivity (92.4 mg l(-1) d(-1)). Similar profile was observed for protein productivity, that achieved a maximum value of 67.0 mg l(-1) d(-1), because of the very high protein content (72.5%) of biomass produced under these conditions. The yield of nitrogen-to-biomass was much higher at the lower No, because of the low protein content. and reached a maximum value of 9.7 g g(-1) at D = 0.08-0.12 d(-1). The yield of nitrogen-to-protein showed less marked difference, being most of the nitrogen present in the cell as proteins or free amino-acids. (c) 2007 Elsevier Ltd. All rights reserved.