Sweet sorghum extract was used as substrate for lipid accumulation by the oleaginous fungus Mortierella isabellina in batch cultures. Various initial sugar (13-91 g/L) and nitrogen (100-785 mg/L) concentrations resulting in various C/N (43-53) ratios were tested. Oil accumulation ranged between 43% and 51% corresponding to oil production from 2.2 to 9.3 g/L. A detailed mathematical model was developed. This model is able to adequately predict biomass growth, lipid accumulation, and sugar and nitrogen consumption. The model assumes that fungus growth is inhibited at high sugar concentrations. A set of kinetic experiments was used for model kinetic parameters estimation, while another set of experiments was used for model validation. The developed model could be generalized for similar systems of lipid accumulation and become a useful tool for reactor design for biofuel production. Biotechnol. Bioeng. 2011;108: 1049-1055. (C) 2010 Wiley Periodicals, Inc.