The high exopolysaccharide (EPS) producing microalga Botryococcus braunii UC 58 produced, under continuous illumination, highs concentrations of EPS (2.5 gl(-1) after 14 days of growth) when nitrate was used as the nitrogen source instead of urea or ammonium (2 mM of nitrogen). In addition, broth viscosity decreased sharply with ammonium or urea when cultures entered the stationary phase, suggesting biopolymer hydrolysis. In the range 0.5-8 mM NO3-, nitrogen appeared to be the growth-limiting nutrient, as suggested by the onset of the stationary phase and the final biomass concentration. Over 30 days of culture, the specific EPS production (EPS produced per unit of biomass) was minimal for 8 mM NO3- and similar for 0.5 and 2 mM, possibly related to the increase in specific EPS synthesis along the exponential phase, reaching the highest levels after growth had stopped. Broth viscosity increased with growth accompanying EPS accumulation but sharply decreased a few days after the cultures entered the stationary phase. This decrease was observed earlier for cultures with lower NO3- concentration. Considering a culture period above 10 days, the increase of NO3- concentration up to 2 mM is recommended to reach higher EPS concentrations, but the use of 8 mM NO3- is only of possible interest for extended growth periods (above 15 days). The introduction of a photoperiod fed to delayed growth and EPS production. However, the specific production is similar when the same phase of growth is considered.