Bi-parental families were produced in hatchery and tested in the field in 2001. Two sets of 5 families were constituted, selected on their high (R) and low (S) survival. These two sets were reared in Brittany from March to November 2002. Samplings were performed twice a month to obtain data on biometry, survival, reproductive cycle, biochemical composition, adenylate energy charge, hemolymph parameters (ions and defense system) and muscle strength. "R" and "S" oysters exhibited different reproductive effort and spawning strategy. "R" oysters allocated less energy in gonad than "S" ones and presented a complete spawning at the end of August contrary to the "S" which spawns partially. Mortality started in July when the seawater temperature reached 19 degree C and affected mainly "S" oysters. Concentrations of Na and Cl ions in hemolymph were different for "S" and "R" from May to June. A bacterial increase in hemolymph (R and S) was observed during the same period. The adenylate energy charge was more lowered for "R" than for "S" oysters, just before the spawning event However, other physiological and immunological parameters were similar between the two sets during the phases of maturation and mortality but discriminated groups after the spawning peak.