The protective effect of selenium against mercury toxicity is well known especially between selenomethionine and methylmercury and it has been studied in several living organisms, however information is lacking about the interaction of these species in Chlorella. Investigation into which chiral form of selenomethionine effectively acts against the toxic effects of methylmercury has not previously been carried out. In the present work, two control cultures and two cultures of C. sorokiniana were grown in standard medium with D,L-SeMet, L-SeMet or D-SeMet. After the experiment was started up MeHg(+) was added periodically to the cultures containing D,L-SeMet, L-SeMet, D-SeMet and to one of the control batches. The results show that both SeMet enantiomers counteract the toxicity of MeHg(+), by markedly increasing the total content of chlorophyll, carotenoids, as well as the dry weight and light dependent oxygen production, compared to the culture which is non pre-treated with SeMet and is only exposed to MeHg(+). The levels of MeHg(+) measured in cells are lower in the cultures pre-treated with SeMet indicating that the passage of MeHg(+) into the cells is negligible when carried out in the presence of SeMet, or that SeMet enhances the release of MeHg(+). On the other hand, L-SeMet is directly involved in the detoxification of MeHg(+), but the involvement of D-SeMet occurs only indirectly since it has been neither identified in the medium nor in C. sorokiniana after supplementation with this enantiomer. It may be that D-SeMet is transformed into SeMeSec and L-SeMet. Moreover, SeMeSec is almost totally released from the cells after 72 hours. No mercury-selenium complex was detected but, since the summation of the different species identified accounted only for 77% of the total selenium and mercury measured directly after sample digestion, it is possible that they are present in the form of an undetected Se-Hg complex. This hypothesis is supported by the decrease of inorganic selenium during the experiment. The present paper reports new data about the relationship between the mechanism of detoxification of methylmercury and selenomethionine enantiomers through the study of the metabolic intermediates by means of speciation analysis.