Summer mortality events have been observed in Pacific oyster Crassostrea gigas for several decades. This paper examines the selective pressure exerted by summer mortality on the polymorphism of a newly identified oyster metallothionein gene. CgMT4 cDNA and genomic sequences were obtained. CgMT4 was studied in two generations of oysters reared in three sites on the French Atlantic coast, using single strand conformation polymorphism analysis. Four alleles were detected. Individuals carrying genotype MT4-CD seem to have higher susceptibility to summer risk conditions. The MT4 gene could be a potential new genetic marker for susceptibility; further validation studies are recommended.