During the non-breeding season, the IIsselmeer area in The Netherlands forms an important staging area for large numbers of diving ducks. The birds feed in the open water at varying distances from the coast and roost either in the lee of dikes or in sheltered waters bordering the lakes IIsselmeer and Markermeer. Most species feed during the day and roost at night. Tufted Ducks Aythya fuligula, Pochards A. ferina and most Scaups A. marila, however, show a reverse rhythm. The IIsselmeer area has been designated as a potential site for wind farms. If wind turbines are placed between the feeding and roosting areas of diving ducks, birds flying at turbine height may run the risk of colliding with the turbines, particularly during darkness. The altitude at which diving ducks fly between feeding and roosting areas is unknown. We studied the nocturnal flight patterns and altitudes of birds crossing dikes and open water between the roosting and feeding areas at six localities in the IIsselmeer area during February and March 1995 (Fig. I, Tab. 1). Flight altitudes were measured from the end of the day until early the next morning using a reconstructed FR 8250 Furuno ship radar. At Andijk, Pampushaven and Lelystad-Haven observations were made close to the roosting areas. At Urkerhoek the birds were about 6 km from the roosting place when they passed the radar. At Oude Zeug and Dijkmagazijn we were able to measure flight altitudes at both short and long distances from the roosting site. At Oude Zeug and Andijk Scaups predominated, at the other sites Tufted Ducks were superior in numbers (Tab. I). At Andijk we also observed Goldeneyes Bucephala clangula and sawbills Mergus spp. Most flights of sawbills and Goldeneyes occurred before sunset and after sunrise. Scaups flew mainly during daylight and twilight (first 45 minutes after sunset and last 45 minutes before sunrise), while Tufted Ducks and Pochards predominantely flew during darkness (figure 2). As a result the majority of nocturnal movements of diving ducks took place during the first one and a half hours after sunset and the last one and a half hours before sunrise. At some localities, we also saw flight movements in the hours in between. In general, it was unknown which species were involved in these nocturnal flights. At Lelystad-Haven, some of the bird echoes originated from roaming gulls. Elsewhere, however, the echoes may have originated from diving ducks wandering at the feeding grounds. Birds predominately passed at altitudes below 75 m (figures 3-6). The few bird echoes at altitudes above 100 m probably originated from migrating geese. When Tufted Ducks and Pochards crossed dikes, they flew at altitudes of up to 75 m, when flying above open water they reached altitudes of up to 50 m. At Urkerhoek, where the ducks were faced with strong head-winds, birds passed at altitudes below 30 m. Present-day middle-sized wind turbines are approximately 70 to 100 m tall. Diving ducks in the IJsselmeer may therefore be at risk when they meet wind turbines on their flight routes between roosting and feeding areas. Since collisions with wind turbines predominantely take place during darkness, Tufted Ducks and Pochards (flight movements mainly during darkness) run a higher risk of colliding than either Scaups (mainly flying between roosting and feeding areas just after sunset and just before sunrise) or sawbills and Goldeneyes (almost exclusively active during day-light hours). In order to estimate the real risk diving ducks run, data on the behaviour of diving ducks actually crossing windturbines on their flight between roosting and feeding areas during darkness are needed. Such a study is presently being carried out in the area.