Previous studies have shown kin recognition abilities in salmonid fish. Some authors have suggested that the attraction of juvenile fish to siblings may indicate preference for shoaling with kin. The aim of the present study is to test the prerequisite for the hypothesis that siblings swim spatially closer than unrelated fish during their seaward migration as smolts. Atlantic salmon (Salmo salar) eggs from three families were each reared in two tanks to create familiar and unfamiliar sibling smolts. Before the experiment started they were tagged individually withpassive integrated transponders (PITs). Twelve individuals from each of six groups were mixed and released together at several occasions in the upper end of the 400-m-long experimental stream. An automatic PIT-monitoring system placed in the outlet recorded the time for passage of each individual leaving the stream. Eighty-five percent of the juveniles monitored by the PIT antenna showed downstream migration at night hours and they migrated significantly more often closer in time to both known and unknown siblings than to unrelated fish. The results suggest that there is a genetic component in the migratory behaviour of Atlantic salmon smolts and support the hypothesis that smolts migrate in kin-structured groups.