Molluscs and especially bivalves are able to accumulate dinoflagelates, diatoms and cyanobacteria toxins, and, being vectors for these toxins, transfer them along food chains. The data obtained from laboratory experiments showed that bivalve molluscs are resistant to cyanobacteria toxins. In this work, we wanted to test if Mytilus galloprovincialis organs react to microcystins and other cyanobacteria compounds by inducing or decreasing its GST activity. Acclimated mussels M. galloprovincialis were exposed to the toxic Microcystis aeruginosa M13 strain. Exposure of mussels to toxins was done in three ways: living Microcystis cells, crude Microcystis extracts and pure toxins. The measurement of soluble and microsomal GST activity in the different mussel organs was done by using the substrates 1-chloro-2,4-dinitrobenzene (CDNB) and 2,4-dichloro-1-nitrobenzene (DCNB). Analysis of the GST activity of the control mussels using CDNB as a substrate showed that cytosolic activity is much more significant than microsomal. Intact M. aeruginosa cells did not induce any significant response from the mussels, showing that these animals are quite resistant to the cyanobacteria if they are intact. On the other hand, cell extracts caused an important effect in the gut, in the gills and in the labial palps, although in different ways. There was an increase in GST activity in the gut and gills of mussels exposed to Microcystis extracts, showing a response of this detoxication pathway, but in the labial palps a severe reduction in GST activity occurred. Pure MC LR+YR induced an increase in GST activity in all organs but the labial palps. The results showed that other substances apart from microcystins may cause stress to mussels and affect detoxication enzymes such as GST.