The three polders of Flevoland give a clear picture of how flora and fauna have developed in newly reclaimed land. The establishment and distribution of butterflies In the polders is described. Among the species to be found are Aphantopus hyperanthus, Pararge aegeria, Quercusia querqus and Anthocharis cardamines. Presence of a source population In the adjacent older land and a similar biotop in the polder is necessary for distribution of non-mobile species. Special mention Is made of the importance of a good ecological infrastructure.