A Chlorella vulgaris UTEX 26 semicontinuous culture was implemented in 2000 L raceways with M medium during spring season at greenhouse conditions. Areal biomass productivities between 20 and 26 g m(-2) d(-1) were reached on the third day. The maximal areal lipid productivity obtained was 6.1 g m(-2) d(-1) and an increment in the saturated fatty acids (SFA) proportion (C14-C18) was favored in comparison with the fatty acids obtained with M medium in photobioreactors of 1 L and photoperiod light: darkness 12:12 h. After the eighth day of the culture or biomass concentrations above 0.25 g L-1, the microalgal cultures were prone to contamination by ciliates and amoebae, due to the sugars excreted by C. vulgaris UTEX 26. The periodical addition of NH4HCO3 to the microalgal culture maintained the ammonium concentration between 25 and 50 mg L-1, which contributed to diminish the contamination risks by protozoa.