This study evaluated an internally illuminated photobioreactor (IIPBR) design for improving biomass productivity through better utilization of light and carbon dioxide-supplementation. Growth of Scenedesmus sp. and Nannochloropsis saline in an 18-L version of this design was evaluated under artificial light at varying CO2-air (vol/vol) ratios, but at constant air supply rate of 0.8 L min(-1) corresponding to gas-to-culture volume ratio of 0.044 min(-1). CO2-air ratios of 4% and 1% were found as the optimal for Scenedesmus sp., and N. saline, yielding volumetric productivities of 0.40 and 0.104 g dry L-1 d(-1), respectively. Under continuous operation with regular harvesting at the optimal CO2-air ratios, constant biomass levels of 1.40 and 0.52 g dry L-1 were maintained with average biomass productions of 2.53 and 0.93 g dry day(-1), for the two species, respectively. Based on volumetric biomass productivity per unit light energy input per unit incident area, performance of this IIPBR design is shown to be comparable to that of bubble column and airlift designs reported in the literature, but at much lower gas-to-culture volume ratio. (C) 2012 Elsevier Ltd. All rights reserved.