Résumé: Seasonal changes of sperm quality which can affect sperm biological parameters throughout the breeding period, have been little studied in mollusc species. Controlling gamete quality would aid the management of gametes in hatcheries and the development of selection programs. The aim of the present study was to describe the changes in sperm quality of wild Pacific oysters through the spawning season by comparing sperm parameters al the beginning (May), middle (July) and end (October) of this period using a panel of bio-descriptors. These parameters were studied over the 2014 breeding season based on shed sperm collected after serotonin injection of wild breeders. A significantly higher percentage of motile sperm was observed al the end of the spawning season (+78% relative to the value observed al the beginning) althought a lower total number of spermatozoa was collected (-59%). The mean condition index of oysters, however, was no different between the three sampling dates. For intratesticular sperm, the increase of the percentage of motile sperm and Velocity of the Average Path (VAP) in relation to time post activation was not significantly different among sperm sampling periods, suggesting that the kinetic of the sperm 'maturation process was similar. Furthermore, the mean VAP observed on shed sperm did not change through the spawning season. The sub-continuous gametogenesis of Pacific oyster can help to explain why only limited consequences of sperm ageing are observed in this species. Furthermore, the effects of sperm ageing may depend on the annual reproductive pattern of Pacific oyster. Statement of relevance: This study showed the effect of sperm ageing on sperm quality parameters. This knowledge is useful for aquaculture and would support the recent trends of mollusc farming, allowing a better management of the gametes in hatchery. (C) 2016 Elsevier B.V. All rights reserved.