This article describes the enrichment of the fresh-water green microalga Chlorella sorokiniana in selenomethionine (SeMet). The microalga was cultivated in a 2.2 L glass-vessel photobioreactor, in a culture medium supplemented with selenate (SeO42-) concentrations ranging from 5 to 50 mg L-1. Although selenate exposure lowered culture viability, C. sorokiniana grew well at all tested selenate concentrations, however cultures supplemented with 50 mg L-1 selenate did not remain stable at steady state. A suitable selenate concentration in fresh culture medium for continuous operation was determined, which allowed stable long-term cultivation at steady state and maximal SeMet productivity. In order to do that, the effect of dilution rate on biomass productivity, viability and SeMet content of C sorokiniana at several selenate concentrations were determined in the photobioreactor. A maximal SeMet productivity of 21 mu g L-1 day(-1) was obtained with 40 mg L-1 selenate in the culture medium. Then a continuous cultivation process at several dilution rates was performed at 40 mg L-1 selenate obtaining a maximum of 246 mu g L-1 day(-1) SeMet at a low dilution rate of 0.49 day(-1,) calculated on total daily effluent volume. This paper describes for the first time an efficient long-term continuous cultivation of C sorokiniana for the production of biomass enriched in the high value amino acid SeMet, at laboratory scale. (C) 2013 Elsevier Ltd. All rights reserved.