The Walenbos is one of the most important forest ecosystems in Flanders and used to harbour 43 butterfly species. Changes in forest management and agriculture has led to a decrease of the species to 27. Afforestation of moist woods and grasslands caused changes in biotops which in turn led to the disappearance of many butterfly species. This is illustrated by an example of the loss of the Small Pearl-bordered Fritillary (Clossiana selene). Conservation of butterflies should he a top priority in the Walenhos. This can he achieved by improved management measures and possibly, in the future, reintroduction of species.