On a tidal flat area in the Dutch Wadden Sea near the Island of Texel, 2-year old Ensis directus (= E. americanus) showed mortality in February 2003, whereas 3 and 5 year old specimens died early May. Mass mortalities of this invader occur irregularly, mainly in late winter – early spring. Both mortalities in 2003 may be related to a cold period in winter, the dying process may have been shorter in the younger cohort.