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-The common whelk Buccinum undatum (Neogastropoda, Buccinidae) is widespread along the north Atlantic coasts of France. In west Cotentin (Normandy), an important local fishery exists that depends exclusively on this marine resource and represents about 90% of French whelk production. Despite successive restrictive measures, fishing yields have decreased since 2000. Shell length frequency distributions of Buccinum undatum caught in west Cotentin were studied in 2005 (N = 2910), in 2007 (N = 2032) and in 2009 (N = 4145) ranging from 7 to 85 mm. In order to preserve this fishery and to characterise the whelk stock at this site, the reproductive biology of this species was studied in relation to age, sex and size, for whelks of commercial size (45 to 61 mm) over the complete annual cycle in 2008. Age was determined by reading the number of operculum striae: animals between 45 and 59 mm had 2 to 4 striae. Four reproductive stages were defined for males and females on the basis of histological analysis. The time course of reproductive events was described according to size and sex: the highest percentages of ripe stages were found in October for both sexes. Spawning events occurred between October and December. The shell length that corresponded to 50% of animals having mature gonads (49 mm and 52 mm for males and females, respectively, corresponding to 3 and 4 years old animals) exceeded the minimal landing size (45 mm). Fifty percent maturity was reached for males in their third year and for females in their fourth year. Reproductive investment was evaluated by quantitative analysis of the gonadal thickness or surface and was found to be proportional to shell size. We found that less than 20% of females under 49 mm in length were sexually mature and that these produced only a few eggs. This information, collected in a major fishing area, should assist fishery management decisions.