In the Kuinderbos forestry (1100 ha) planted 26-31 years ago in the N.E, Polder an ecological investigation of the ferns was carried out in 1979. The 23 species found represent nearly the complete Dutch fern flora; two species are new to The Netherlands. The establishment of so many species reflects the great dispersal capacity of the group and seems to be evoked by the presence of a great length of ditches (240 km), opening various types of soil under different tree species. The ditch environment markedly differs from the known habitat of many of the species in NW Europe. Correlations between species and environmental factors (e.g. pH, organic matter, lime content and light intensity) were in good agreement with literature data; their ecological amplitude seems to be widened in this young forest.