Batch cultivations were performed to evaluate the influence of phosphate concentrations (0.25, 0.5, 0.75, and 1.0 g L(-1)) for pH regimes (9.5, 10.0, and 105) on the biomass production by Spirulina platensis. The best condition for cell growth (3.099 g L(-1)) was found at 0.5 g L(-1) phosphate and pH value of 10.0. Cultivation time, phosphate, and pH caused to increase significantly (p < 0.01) in biomass production by S. platensis. Lag time was observed up to 4 h. After then, biomass production increased sharply (p < 0.01) from 0.020 g L(-1) to 2.063, 2.213, 1.532, and 0.797 g L(-1) at 0.25, 0.5, 0.75, and 1.0 g L(-1) phosphate values, respectively. Modified Gompertz model could be regarded as sufficient to describe the biomass production by S. platensis with high determination coefficients and low sum of square value indicated that. Biological parameters for biomass production were successfully predicted by modified Gompertz model. (C) 2009 Elsevier Ltd. All rights reserved.