In a bubble column reactor with an inner diameter of 6 cm and a height of 63 cm for the culture of cyanobacteria two different shapes of bubbles can be generated, resulting in bubbly flow or slug flow. Growth of Anabaena variabilis under slug flow (1.9 g/l/day) was 1.73 times higher than that under bubbly flow (1.1 g/l/day) when the specific irradiation rate was maintained above 10 mu mol/s/g dry cell. Although a stepwise increase in superficial gas velocity enhanced the average cell growth rate under bubbly flow by 1.57 times. the average cell growth rate during the deceleration phase under bubbly flow (1.98 g/l/day) was 0.61 times smaller than that under slug flow (3.22 g/l/day). These results demonstrate that the bubble shape in the slug flow was advantageous in regards to the radial circulation of cells. (C) 2012 Elsevier Ltd. All rights reserved.