Several bat species are known to migrate long distances between summer and winter roosts. During migration, many bats even cross the North Sea. The developments of offshore wind farms in the North Sea could therefore pose a collision risk for migrating bats. While bats have been observed inside offshore wind farms, their activity at turbine rotor height yet remains unknown. We therefore installed acoustic bat detectors at wind turbines in the Belgian part of the North Sea. Seven detectors were installed on the service platform of the transition piece (16 m above mean sea level) and four were installed on the nacelle of the turbines, in the centre of the rotor swept area (93 m above mean sea level). A total of 151 recordings of call sequences of Pipistrellus nathusii (Nathusius' pipistrelle) were made during 20 nights over an entire autumn migration season (8 August – 30 November 2017). 45 recordings contained more than 10 calls. These were further investigated for behavioural clues. We identified 32 recordings of animals in transit and 10 sequences of animals passing by while simultaneously exploring. Only three detections contained feeding buzzes and/or intense exploratory behaviour. The number of recordings at 93 m were around 10% of the number of recordings made at 16 m. This indicates that the activity of P. nathusii at our study site, measured at that particular altitude is low. Our observations therefore suggest that the collision risk might be lower than what could be expected from low altitude observations. However, a low number of recordings at nacelle height does not necessarily mean that only a low number of bats will collide with the turbines. The activity in the outer parts of the rotor swept zone, outside the detection range of our acoustic detectors, remains unknown and should be further investigated.