The micronucleus assay in gill tissue of the mussel Mytilus galloprovincialis has been developed in our Laboratory to assess the mutagenic activity of compounds present in marine environments. The sensitivity of the test was assessed by performing mutagenic treatment for 48 h with the two standard compounds vincristine (VCR) (0.005, 0.01, 0.02 mg l-1), and benzo(a)pyrene (BaP) (0.025, 0.075, 0.225, 0.675 mg l-1). Since both tested chemicals produced significant increases in the number of micronucleated cells, animals were directly exposed to marine waters: mussels grown in clean water (control sample) were transferred to polluted areas and then collected weekly. Micronuclei frequencies of sampled mussels were significantly higher than the value of the control group. © 1990.