The mosses of the Wadden Sea islands Rottumerplaat, Rottumeroog and Zuiderduin (the Netherlands, province of Groningen) Rottumerplaat, Rottumeroog, and Zuiderduin are three small barrier islands in the Dutch part of the Wadden Sea. Rottumerplaat is the largest and is still becoming larger. It is relatively young and came into existence as a sandbank in the nineteenth century. Since 1950, it can be considered a true island. Rottumeroog is the oldest one. Since the sixteenth century, it is moving eastwards and, simultaneously, it is becoming smaller. Zuiderduin is the smallest and the youngest of the islands. Since it separated from Rottumeroog in c. 1930, it is rapidly moving southeastwards. In 1999, an inventory was made of the bryoflora of these islands as part of a monitoring project of the island flora, which since 1996 is carried out by Staatsbosbeheer regio Groningen-Drenthe. This paper presents the recent findings and is supplemented with a historic overview of earlier records of bryophytes for Rottumerplaat and Rottumeroog; the bryoflora of Zuiderduin was previously unknown. The inventory resulted in a list of 34 bryophyte species on Rottumerplaat, 24 species on Rottumeroog, and 8 species on Zuiderduin. During the last c. 25 years, the number of species on Rottumerplaat has increased by 12 species, whereas the number of species on Rottumeroog has decreased by 16 species. The most interesting new species on Rottumerplaat are Dicranum scoparium, Pleurozium schreberi, Rhytidiadelphus triquetrus, Homalo-thecium lutescens and Orthotrichum cupulatum (1 tiny specimen; new for the Dutch Wadden Sea-area). D. scoparium is also new for Rottumeroog. R. triquetrus was found on this island in 1869 and has never been found since until 1999. The bryologically most interesting site on Rottumeroog (" tuin van Toxopeus ") has disappeared by floods in 1998–1999 together with, amongst others, the rare species Moerckia hibernica en Bryum calophyllum. Inleiding In 1999 is, met een grondige inventarisatie, een begin gemaakt met het monitoren van de mosflora van Rottumerplaat, Rottumeroog en het Zuiderduin (Fig. 1), drie kleine, tot de provincie Groningen behorende waddeneilanden, die samen ook wel Rottum worden genoemd. De namen Rottumeroog en Rottumerplaat zijn algemeen bekend.