AbstractBeluga whales (Delphinapterus leucas) inhabiting the St. Lawrence estuary are highly contaminated with environmental pollutants and have a high incidence of cancer. Environmental contaminants may be partly responsible for the high incidence of cancer observed in this population. DNA damage plays an important role in the development of cancer. The micronuclei assay was used to test the genotoxic potential of mercury compounds in skin fibroblasts of an Arctic beluga whale. Both mercuric chloride (Hg) and methylmercury (MeHg) induced a highly significant (p < 0.001) dose–response increase of micronucleated cells. Statistically significant increases in micronucleated cells were observed for 0.5, 5, and 20 μg/ml Hg and 0.05, 0.5, and 2 μg/ml MeHg when compared to control cultures. Concentrations of 0.5, 5, and 20 μg/ml Hg induced a two-, three- and fourfold increase of micronucleated cells, respectively. Treatment with MeHg was one order of magnitude more potent in inducing micronuclei and in inhibiting cell proliferation than Hg. Although results of this in vitro study do not imply that mercury compounds are involved in the etiology of cancer in St. Lawrence beluga whales, significant increases in micronuclei frequency were found at low concentrations of MeHg (0.05 and 0.5 μg/ml) that are believed to be comparable to concentrations present in certain whales of this population.