This paper presents a model for the continuous production of green cells of the microalga Haematococcus pluvialis, in both indoor and outdoor conditions. To develop this model, the influence of irradiance and dilution rate on the performance of continuous cultures of H. pluvialis was studied in the laboratory but simulating outdoor conditions. Characterization of the cultures included biomass productivity, fluorescence of chlorophylls, pigment content, elemental composition of the biomass, cell density, cell size, homogeneity and nitrate consumption rate. Results showed that the optimal dilution rate was 0.04 h(-1), and that higher external irradiance resulted in higher biomass productivity in all cases, with a maximum value of 0.58 g L-1 day(-1). Continuous cultures were stable for more than 3 months, in spite of photoinhibition at noon, producing homogeneous biomass with a stable biochemical composition and cell morphology at each steady state. Astaxanthin accumulation was not observed in spite of the high levels of irradiance essayed, and cells remained in the flagellated-palmeloids green form whatever the culture conditions. High dilution rates produced small cells of 22 p,m diameter, with a high nitrogen content of up to 10.0% d.wt. The average irradiance within the reactor was the main factor determining the behaviour of the cultures, although the external irradiance impinging on the reactor surface also influenced the results, indicating the existence of photoinhibition. The influence of both external and average irradiance on the growth of H. pluvialis was modelized. The accuracy of the model obtained was verified on a 0.22 m(3) outdoor tubular photobioreactor operated in both discontinuous and continuous mode, obtaining a maximum biomass productivity of 0.68 g L-1 day(-1). The model reproduced the experimental data of biomass concentration and productivity, cell size and nitrate consumption, providing to be a powerful tool for optimizing the design and operation of outdoor photobioreactors for the production of green cells of H. pluvialis. (c) 2005 Elsevier Inc. All rights reserved.