Cellular growth and intracellular concentrations of selenite in the unicellular green alga Chlamydomonas reinhardtii showed considerable variability between replicates at ambient concentrations of Se that became toxic to the alga. However, growth inhibition was well correlated to the cellular quota of selenite; the correlation between absorbed selenite and toxicity was better than the simple correlation between ambient selenite and toxicity usually used to express metal toxicity, suggesting selenite toxicity is mainly linked to intracellular accumulation. It is usually expected that effects of contaminants on organisms depend on internal concentrations. These results confirm this hypothesis.