Many former seadikes in the Netherlands have lost their contact with salt water due to the enclosure of the Zuiderzee and the Deltaworks. These dikes now constitute a suitable habitat for ferns. The common occurrence of seven fern species between heavy blocks of granite and basalt on dikes along the Zuiderzee and Lauwerszee in Friesland is recorded. Outside Friesland three or four other fern species have been found on former seadikes.