The variation in the colony size distribution with time of microalga Botryococcus braunii, which produces extracellular polysaccharides as well as high levels of liquid hydrocarbon, was measured in bubble column photobioreactors. The effect of average light intensity on the size of colonies was examined by fixing films impervious to light on the column wall and measuring the light intensity within the photobioreactors. The colony size distribution of this alga was found to fit the log-normal distribution. Under identical hydrodynamic conditions, the average colony size shifted to an equilibrium size, which was determined depending on the average light intensity within the photobioreactors.