Several European bat species migrate long distances of >1,000 km, but information on migration routes is poor or anecdotal, or both. We investigated migration of bats along the Oder River valley in southwestern Poland to determine the significance of large river valleys as migration corridors for bats. We determined direction of bat movements from visual observations and sequence of echolocation signals received by 2 bat detectors, from March to November 2007. The predominant activities of bats were directional flights in spring (March–May) and autumn (mid-August–October) and foraging in summer (June–July). In spring, most bats flew northward, and in autumn, southward. In autumn, the movement occurred over a longer time and there were fewer bats migrating than in spring. We conclude that river valleys are migration flyways for bats that travel long distances and those that travel short distances, and that differences between spring and autumn migration may be related to food supply, to energy demands, to seasonally different routes, or to a combination of these factors.