The requirements for efficient utilization of high light fluxes in cultures of Spirulina platensis have previously been elucidated. The most important of these was a short light-path coupled with a highly turbulent flow, facilitating ultrahigh cell densities (i.e. above 100 mg chl l−1). The present study shows that for each irradiance there is an optimal culture density, defined as the concentration that yields the highest output rate of cell mass under the prevailing conditions. In ultrahigh cell density cultures, a linear relationship was observed between the output rate and the irradiance, up to a photon flux density (PFD) of 2500 μmol m−2 s−1. Using a total PFD of 8000 μmol m−2 s−1, a maximal output rate of 16.8 g dry weight m−2 s−1 was obtained, which is the highest reported for a culture of photoautotrophic microorganisms exposed to direct beam radiation. Testing the effect of reduction in light-path on productivity, output rate per unit volume increased 50-fold as the light-path was reduced 27-fold, i.e. from 200 mm to 7.5 mm. Likewise, the output rate calculated on an areal basis was almost doubled, increasing from 2.5 to 4.6 g m−2 s−1, and the specific growth rate increased c. 20% as the light-path was reduced to 7.5 mm. The very significant effect of the length of the light-path on enhancing the output rate is interpreted as resulting from improvement in the light regime to which the individual cells are exposed, as reflected in the frequency and overall characteristics of the light–dark cycles.