The viability of large-scale microalgae cultivation depends on providing optimal growth conditions, for which a key operational parameter is culture density. Using Synechocystis sp. PCC 6803, we conducted a series of fixed-density, steady-state experiments and one batch-growth experiment to investigate the role of culture density on biomass production and light utilization efficiency. In all cases, the fixed-density, steady-state experiments and batch-growth experiment showed good agreement. The highest biomass production rates (260 mg L(-1)d(-1)) and efficiency for converting light energy to biomass (0.80 g (mol photons)(-1)) occurred together at a culture density near 760 mg L-1, which approximately corresponded to the lowest culture density where almost all incident light was absorbed. The ratio of OD680/OD735 increased with culture density up to the point of maximum productivity, where it plateaued (at a value of 2.4) for higher culture densities. This change in OD680/OD735 indicates a photoacclimation effect that depended on culture density. Very high culture densities led to a sharp decline in efficiency of biomass production per photons absorbed, likely due to a combination of increased decay relative to growth, metabolic changes due to cell-cell interactions, and photodamage due to mixing between regions with high light intensity and zero light intensity.