© Triveni Enterprises, Lucknow (India). Aim: To potentially improve the productivity of microalgae for mass culture, growth characteristics and light-saturation index (Ek) in the marine microalga Chlorella vulgaris were investigated under high-cell-density conditions grown in batch cultures. Methodology: Potential changes in the growth characteristics and Ek in C. vulgaris were investigated during 4 weeks of cultivation. The algae was cultivated in a column reactor at 25°C under a surface light intensity of 300 μE m-2s-1 in a 12-hr light and 12-hr dark cycle. Results: Cell density increased and reached 2.7 g-dw l-1 by the end of culture with an areal production rate of 3.3 g-dw m-2day-1. The specific growth rate reached a maximum of 0.41 day-1 when the light intensity per cell was 7.9 μE g-dw-1 s-1. As the light intensity per cell decreased with increasing cell density, Ek decreased gradually during the first half of the culture period. Interpretation: Chlorella vulgaris in this study had the ability to acclimate to shade conditions during high-cell-density cultivation, however, the photosynthetic efficiency decreased. High transformation efficiency of light energy for photosynthesis in shade-acclimated cells will help to exploit the mass culture of microalgae.