The microalgae Chlorella vulgaris produce lipids that after extraction from cells can be converted into biodiesel. However, these lipids cannot be efficiently extracted from cells due to the presence of the microalgae cell wall, which acts as a barrier for lipid removal when traditional extraction methods are employed. Therefore, a microalgae system with high lipid productivity and thinner cell walls could be more suitable for lipid production from microalgae. This study addresses the effect of culture conditions, specifically carbon dioxide and sodium nitrate concentrations, on biomass concentration and the ratio of lipid productivity/cellulose content. Optimization of culture conditions was done by response surface methodology. The empirical model for biomass concentration (R2=96.0%) led to a predicted maximum of 1123.2mgdwL-1 when carbon dioxide and sodium nitrate concentrations were 2.33% (v/v) and 5.77mM, respectively. For lipid productivity/cellulose content ratio (R2=95.2%) the maximum predicted value was 0.46 (mglipidL-1day-1)(mg cellulose mg biomass-1)-1 when carbon dioxide concentration was 4.02% (v/v) and sodium nitrate concentration was 3.21mM. A common optimum point for both variables (biomass concentration and lipid productivity/cellulose content ratio) was also found, predicting a biomass concentration of 1119.7mgdwL-1 and lipid productivity/cellulose content ratio of 0.44 (mglipidL-1day-1)(mg cellulose mg biomass-1)-1 for culture conditions of 3.77% (v/v) carbon dioxide and 4.01mM sodium nitrate. The models were experimentally validated and results supported their accuracy. This study shows that it is possible to improve lipid productivity/cellulose content by manipulation of culture conditions, which may be applicable to any scale of bioreactors. Biotechnol. Bioeng. 2013; 110: 2114-2122. (c) 2013 Wiley Periodicals, Inc.