In this paper, the occurrence of a population of Water Rails (Rallus aquaticus L.) on the Waddensea-island Vlieland is discussed. Water Rails are three years periodical localized on sound and trapped by riddle claptrap (De Kroon 1979). From 1976-1978, the number of the breedingspairs was respectively 19, 21 and 27. Where as in 1979 only 5 pairs were found (figure 1). The results of capturing are summarized in table 1 and figure 3.