The use of oyster gills for the analytical determination of metallothionein (MT) concentration as a biomarker of metal exposure was investigated. Temporal variations in MT and metal concentrations (which can interfere with inter-site differences) were examined over a 7 month period (from spring to autumn) in Japanese oysters from a clean site (Bay of Bourgneuf, France) and a metal-rich site (Gironde estuary, France) as well as in individuals translocated from the clean to the contaminated area. The ratio between the annual average of MT concentrations in specimens from the clean and the metal-rich sites was 1.3. During the last 3 months of the experiment, significant differences were no longer registered between transplants and residents from the Gironde estuary. Metals concentrations in oyster gills differed consistently between the clean and the metal-rich sites (annual average ratios of 1.5, 2.7 and 9.8, respectively, for zinc, copper and cadmium) and a fast increase in metal concentrations (over a few months) was observed in transplants, mainly for cadmium. MT and soluble metal concentrations were found to be positively and significantly correlated over the period of the study. This relationship is a positive argument for a possible use of gill MT concentration as a biomarker of metal pollution in contrast to previous findings on the digestive gland, there being a smaller amount of seasonal variability in the weight of oyster gills.