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Navicula salinicola is a marine microalgae belongs to Bacillariophyceae which has prospects as a source of bioenergy stocks and valuable bioactive molecules. High biomass productivity is one of the main aims to achieve optimum yield in microalgae cultivation. Biomass production during autotrophic growth of N. salinicola in photobioreactors is governed by environmental factors including CO2 as carbon source during photosynthesis. Also, the amount of harvested microalgae determines the ability of microalgae culture to maintain high growth rate in a continuous or semi-continuous cultivation system, thus affects total biomass yield. The aim of this study was to investigate CO2 concentrations (ambient, 10% and 20%) that produce optimum N. salinicola biomass in photobioreactors, at two different levels of harvesting, i.e. 10 and 20% of culture volume during 11 day of growth. N. salinicola was cultivated outdoor in vertical tubular photobioreactors. Injection of 20% CO2 into the cultures that were harvested 10% daily produced the highest number of cells and biomass (4.2 x 106 cells/ml and 1.7 g/l/day, respectively). The highest total biomass production during 11 day of growth was achieved in the culture injected with 10% CO2 and harvested 10%. This study suggested that CO2 application and the amount of culture harvesting have an effect on the biomass productivity of N. salinicola. [ABSTRACT FROM AUTHOR]