Bench-scale biocalorimetry (=1 L) allows for the determination of the metabolic heat flow during bioprocesses under complete control of all process conditions for extended periods of time. It can be combined with a number of on-line and off-line measurement techniques. This combination can significantly improve insight into the metabolism of microorganisms and the optimization of bioprocesses. In this study it is demonstrated that bench-scale biocalorimetry can also be applied to phototrophic microorganisms. The green microalga Chlorella vulgaris CCAP 211/11B was cultivated in a Mettler-Toledo RC1 calorimeter adapted for high-sensitivity biological calorimetry (BioRC1). Heat production was monitored in 1.5 L batch cultures. In the linear phase of growth, inhibitors of photosynthetic electron transport (DCMU, 3-(3,4-dichlorophenyl)-1,1-dimethylurea, and DBMIB, 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone), were used to stop photosynthesis and to monitor the resulting increase in the energy dissipating heat flux. This resulted in a calculated storage of light energy as chemical energy, i.e. biomass, of 141 ± 12.2 mW L-1 (±S.D.). In addition, it was demonstrated that calorimetric determination of the total amount of light energy absorbed within the reactor was accurate by comparing two different calorimetric techniques. Using both the value of the total light input and the quantity stored as chemical energy, the photosynthetic efficiency could be calculated as 10.5% in this example.