Abstract From chemostat cultures of the marine microalga Isochrysis galbana (CCAP 927/15) simultaneous photolimitation and photoinhibition was observed. The extent of each phenomenon depends on the light gradient inside the culture and therefore on the incident irradiance. Variations in biomass concentration and average irradiance inside the culture with dilution rate at three incident irradiances, I o , were studied (from I o = 820 to 3270 μE m −2 s −1). At I o above 1630 μE m −2 s −1 a photoinhibition effect was observed, although the specific growth rate remained a hyperbolic function of average irradiance regardless of incident irradiance. To calculate average irradiance, a three-dimensional irradiance distribution model for cylindrical geometry is proposed, improving the estimation of the irradiance field inside culture with regard to other methods used up to now since the variations in illumination along the vertical axis are considered. Lastly, a new approach to model simultaneous photolimitation and photoinhibition is proposed by considering that specific growth rate is related to average irradiance and that parameters representing the cell adaptability to light are a function of the maximum irradiation at which cells are exposed, that is, the incident irradiance.