In the autumn of 2012, we conducted a pilot study with ultrasonic recorders to assess the occurrence of bats over the North Sea. At Offshore Wind Farm Egmond aan Zee (OWEZ) a recorder was installed at the meteorological mast and at Princess Amalia Wind Farm (PAWP) a recorder was attached to the entrance platform of an offshore wind turbine. There were 189 recordings of bat echolocation calls at OWEZ and 25 at PAWP. Virtually all recordings concerned Nathusius’ pipistrelle (Pipistrellus nathusii); noctule (Nyctalus noctula) was noted a few times. Bats were only recorded during nights with low or moderate wind speeds, no precipitation and a high ambient pressure. It seems unlikely that the observations referred to individuals which were blown off course by storms, and there are no indications that roosts were present in the vicinity of the recorders. The occurrence pattern of Nathusius’ pipistrelle indicates that the observations were of migrating individuals. The observations of noctule possibly concerned migrants as well, but they could also be residents from the mainland which may use the wind parks as foraging area.