An on-line measurement technique for estimating biomass production rate in a photosynthetic micro-organism culture was developed and tested experimentally. The technique is based on monitoring O-2 production from the increase in pressure inside a closed photobioreactor. The data obtained by this method correlated with the direct measurement of the biomass concentration. A material balance on the components in the system allows the validity domain of the method to be defined. The method was applied to batch cultures of the cyanobacterium, Spirulina platensis, in a cylindrical photobioreactor validating existing physiological and light energy models.