Ecology, syntaxonomy and population size of Gymnocarpium dryopteris have been studied, especially during the period 1979–1991 by collecting data on 419 colonies in the Kuinderbos (a planted woodland in the Noordoostpolder, a polder reclaimed from the former Zuiderzee in 1942) and 74 colonies elsewhere in the Netherlands. In this paper the Kuinderbos population is compared with other Dutch populations (pleistocene area) in order to provide more insights in the ecology of the species and circumstances facilitating high density. Populations in Germany were studied as reference. In the period 1979–1991 the species was recorded in the Netherlands with at least 670 colonies, of which 89% are in the Kuinderbos. Gymnocarpium dryopteris prefers shadowed ditches and drainage trenches, but can grow at various other habitats as well. Especially at the western and southern part of the Netherlands, the species grows on canal walls. Sites with Gymnocarpium dryopteris belong to different communities. In the Kuinderbos stands of Picea sitchensis are preferred, while elsewhere in the Netherlands the species prefers the Quercion roboris-petreae (Fago-Quercetum) or Pseudotsuga mensiezii-stands. Gymnocarpium dryopteris is accompanied by 14 fern species, at the Kuinderbos by 12 fern species, with Athyrium filix-femina as characteristic species, often indicating potential sites. Rhizomes are shallow creeping, at average 2.6 cm below the surface (0.5–8 cm). It grows in the ecto-organic layer or just below this layer. At trench sides it often grows in the mineral layer. Most colonies are within the influence of the watertable. The Dutch colonies are small and colonies with more than 1000 fronds are rare. This probably indicates that most colonies are less than 20 years old. Colonies with more than 100 fronds are often fertile, with less than 20% of the fronds bearing sori. At the wall habitat small-sized individuals may be fertile as well. The high density in the Kuinderbos can be attributed to an optimal water supply from a peat subsoil, the composition of the ecto-organic layer consisting of Picea needles and an optimal light climate. The high density in Picea sitchensis within the Kuinderbos is unprecedented in Europe and makes these stands resemble the natural habitat of Picea sitchensis at the west coast of N. America.