The biofixation of carbon dioxide (CO2) by microalgae has been proven to be an efficient and economical method, mainly due to the photosynthetic ability of these microorganisms to use this gas as a source of nutrients for their development. The aim of this work was to study the growth of Spirulina LEB18 and Chlorella kessleri microalgae, exposed to controlled and non-controlled conditions, with the injection of different concentrations of CO2. The cultures was carried out in 6 L open raceway ponds, under controlled conditions at 30 degrees C and 39 mu E m(-2) s(-1) and under non-controlled conditions, protected by a tunnel of transparent film. The experiments were subjected to CO2 injections at concentrations of 0.038, 6, 12 and 18% (v/v). The highest concentration of biomass (4.95 g L-1) and maximum daily fixation (0.21 g g(-1) d(-1)) were obtained for Spirulina LEB18 in culture that was prepared in non-controlled conditions with an injection of 6% (v/v) of CO2. C. kessleri had maximum (p < 0.0008) specific growth rate (0.84 d(-1)) when grown with 18% (v/v) of CO2 in non-controlled conditions of cultivation. (C) 2011 Elsevier Ltd. All rights reserved.