The diploma thesis consists of a synopsis of the biology of the Fenton’s Wood White (Leptidea morsei), its distribution in Slovenia, the reasons for its decline in distribution and recomendations for the conservation of this species. The Fenton’s Wood inhabits clearings at the sunny edges of forests and clearings in deciduous and mixed forests. It is one of the most endangered butterflies in Europe. In Slovenia its development is linked to the plant Lathyrus niger, as the female lays the eggs on this plant. Its existence is threatened primarily by the changing forest management, early mowing and harvesting of herbal vegetation, habitat fragmentation and climate change. Using the collected data from three time periods (until 1990, between 1990 and 2005, and after 2005), we investigated the actual distribution and range of the species in Slovenia. The Fenton’s Wood White`s full distribution area in Slovenia is in the regions of Kočevje, Bela krajina, Suha krajina, Gorjanci, Krško, Bizeljsko, Podravje, river Kolpa valley and the soughtern parts of Posavje region. The results showed that the range of the species is decreasing. In contrast, to these results the data collected from Centre for Cartography of Fauna and Flora suggests that the actual distribution is increasing. The reason for this putative increase is linked to bigger effort in mapping of the species distribution; consequently, more information is available. Endangerment could be reduced by reforming the forest management. The key measure is to maintain enough forest areas where there are sufficient clearings, since the clearings are the natural habitat of the plant Lathyrus niger. Another essential factor is that the first mowing of the herbal vegetation alongside forest edges should be moved to September. Since butterflies represent an indispensable part of the ecosystem it is imperative to make an effort to welcome these measurements and enable this butterfly species to survive.