Due to an ever-increasing urbanisation, industrialisation, development of road infrastructure and an intensive agriculture, forests in Flanders have become heavily fragmented. In general, organisms bound to small forest fragments have a reduced population size and are highly isolated from other populations. To assess the population genetic effects of forest fragmentation, we chose Coelotes terrestris (WIDER, 1834) as a model organism, because it is strongly bound to forest habitats. A first attempt to reveal the population genetic structure of this species was made by using allozyme electrophoresis. Only one enzyme (PGI) however showed good interpretable variation. This low degree of polymorphism together with the sometimes-questioned neutrality of allozyme markers made us choose genetic marker (RAPD). Ten forests, with a variable degree of isolation and a variable size were investigated. The majority (allozymes) and all (RAPD) pairwise comparisons of population allele/marker frequencies were significantly different, implying a very high degree of genetic isolation between the spider populations inhabiting the forests. No significant correlation could be found between the genetic diversity of the populations and the size of the forest in which they predominate.