Data were collected at one of the important stop-over sites in the southern Baltic between 1989 and 1995. Sandwich Terns were not only more advanced in primary molt than Common Terns, but also replaced their flight feathers faster due to larger number of primaries growing simultaneously. All caught adult Sandwich Terns were in active molt, whereas over 21% Common Terns either had not started primary molt or suspended it. The difference in migration strategy may be the main reason for the discrepancy between these two species in molt advancement. Biometrical analysis showed that juveniles of both species did not reach adult size at this stage of migration. Sandwich Terns caught in late summer in northeast England had shorter wings than birds from Puck Bay probably due to different origin of birds caught at these two sites. Common Terns caught in Puck Bay were larger than birds from German and Scottish breeding colonies and also from birds measured in northeast England. Moreover in Puck Bay, 84% of Common Terns were replacing primaries during autumn migration, whereas only 45% in Teesmouth did it at the same time. It seems that Common Terns from west Europe and terns passing the Baltic Sea belong to different populations, which differ in the distance they cross between breeding and wintering grounds and also in biometrics.