Elements of the mollusc fauna of the island of Marken in the former Zuiderzee (Cl ca 4000 mgr/l) were mentioned by VAN NIEUWENHOVEN, 1942, and by VAN BENTHEM JUTTING, 1944. Since then the island has been connected with the mainland by a dike in 1959. The author had an opportunity to visit the former island May the 15th 1959 and studied the mollusc fauna. Phytia myosotis has disappeared. The newly immigrated freshwater species already recorded by VAN NIEUWENHOVEN and VAN BENTHEM JUTTING are: Physa fontinalis, Lymnaea stagnalis, L. palustris, L. ovata, Planorbis planorbis and P. crista. Valvata piscinalis, Bithynia tentaculata, Lymnaea truncatula and Planorbis vortex are new additions to this list of immigrants. Hydrobia stagnorum was found dead, buried in the mud of inland ditches. This species, though not recorded for the island, must have lived there in the days of the former Zuiderzee. The only land molluscs found belong to Succinea elegans. This is also a new record. It might be possible that colonization by land molluscs is difficult because artificial fertilizers are used on the meadows. Empty valves of inhabitants of the former Zuiderzee: Cardium edule forma paludosa and Mya arenaria were found washed ashore among large quantities of Dreissena polymorpha and Anodonta piscinalis. Some very fresh entire shells belong to Pseudanodonta complanata. These three species form elements of the new IJsselmeer fauna, successors to Cardium, Macoma, and Mya from the former Zuiderzee. The Cl contents of the island’s inland waters are relatively high (300-500 mgr/l in summer and 500-1000 mgr/l in winter and spring) caused by a salt water spring which also influences the Cl values in some other parts of the province of Noord-Holland and the Gouwzee. The island therefore remains a brackish water reserve. A good explanation for the strange seasonal fluctuation of the Cl values could not yet be found.