According to scarce available data, the use of coastal habitats by bats appears to be the most intensive during spring and autumn migrations, when coasts become migratory corridors. Migrating bats might seasonally enrich the fauna of sea islands and peninsulas, which often are poor in bats outside the migratory season. Hel Peninsula is part of the Polish Baltic Coast that extends the most into the sea; thus, it gives unique opportunity to assess that phenomenon quantitatively. To test for seasonal variation in structure of bat assemblage and bat activity, we recorded echolocation calls on walking transects. Seven species were recorded in total, with predominance of sedentary Eptesicus serotinus in June-July and migratory Pipistrellus nathusii in August-September. Occurrence of bats was the highest in deciduous and coniferous forests and the lowest in coasts and ports. Interaction of day of year and distance to mainland was highly significant, indicating that occurrence of bats was increasing from spring to autumn but this increase depended on location. Far from mainland, the mean occurrence of bats was rather stable over time. On transects located close to the mainland, bat occurrence was low at the beginning of the season but after midAugust increased rapidly. We confirmed that some topographically isolated parts of the sea coasts can host only very impoverished summer bat fauna. Bat activity in such sites increase in autumn; however, Hel Peninsula presumably does not act as a migratory corridor, but only as a target of local dispersal from mainland.