Introduced for aquaculture purposes, Ruditapes philippinarum has naturally recruited in several areas along the French Atlantic coast, and is subject to recurrent mortalities in late winter. The possible impact of these mortalities on both newly settled spat and juvenile R. philippinarum, was examined in a population from the Bay of Brest (western Brittany), in parallel with a survey of the main environmental parameters. From February to March 1994, 84% of the juvenile clams, between 1 and 2-y old, disappeared. No direct link could be made between these mortalities and the environmental parameters. Such winter mortalities were not observed in the newly settled spat.