Investigations carried out in a coastal stream flowing to the Northern Bothnian Sea (Ängerån, 63°35'N, 19°50'E) have shown a high drift rate of mayfly nymphs towards the coastal areas. The nymphal development takes place in the estuaries with low salinity (conductivity between 47 and 9800 μS; salinity between 0 and 4–5%0). After the emergence (May/June) the adults fly from the coastal areas to lay their eggs in the stream biotope in the Ängerån. There thus appears to be a colonization cycle between the coastal stream and adjacent coastal areas. The migration movements of downstream drift and the compensatory upstream directed flight are interpreted as asurvival strategy of the species concerned.