Lipid productivity of Chlorella vulgaris and Nannochloropsis oculata was evaluated in a 5 L lab scale externally illuminated photobioreactor. The effect of operating conditions such as pH, temperature, light intensity and photoperiod on biomass productivity, lipid content and lipid productivity was evaluated. At optimum conditions, a maximum lipid productivity of 69.46 and 192.3 mg lipid. L -1 .day- 1 was observed for C.vulgaris and