The breeding behaviour of a Hen Harrier pair on the island of Rottemeroog is described. Rottemeroog is one of three small islands in the eastern part of the Dutch Wadden Sea (combined area c. 1050 ha, of which 232 ha vegetated). The pair was nesting in the overgrown garden of the former warden, a garden which was -in 1998- on the verge of being swallowed by the sea. During the winter of 1998/99 the major part of the garden disappeared into the sea and the pair did not breed on the island in 1999. The nest contained a completed clutch of 4 eggs on 24 April; the first egg probably hatched on 12 May (start of laying back-calculated at 11 April). On 20 June the first nestling fledged, followed by two others on 25 June; the fourth egg failed to hatch. Two juveniles left for the mainland on 17 July, the third fledgling remained on the island and fed on emaciated Starlings; up to 1000 Starlings roosted on the island from 25 June onwards. The adult female stayed on the island all the time. The adult male also visited Rottumerplaat and Zuiderduin (up to 5 km away), and made prolonged trips (lasting up to 2 days) to the mainland (>l5 km away). The male never carried prey when returning from the mainland. During incubation several territorial conflicts were witnessed. On 25 April, an intruding adult female was chased away by the resident female, for which she had to leave the nest 7 times. A 2nd calender-year male tried to take over the resident female when the resident male was visiting the mainland on 28 April; this immature bird displayed and fed the resident female twice. After the adult male returned at 20.00 hr, both males fought intermittenly for several hours during which they interlocked claws and tumbled on the ground where they stayed for 1.5 hrs. Eventually, the intruder was chased away. During incubation, most prey was caught by the male, although the female hunted during absences of the male (at least once successful). After 15 June, the male stayed only rarely on Rottumeroog. During the nestling stage, 75 prey transfers were witnessed; the prey could be identified in 32 cases. The female took larger prey species than the male (Table 1). Near the nesting site, a plucking of a juvenile Magpie Pica pica was found, and the male was seen trying to capture a Turtle Dove Streptopelia turtur and juvenile Jackdaws Corvus monedula.