The common Adder (Vipera berus) is a species in sharp decline in Europe, and relictpopulations are subject of major conservation planning in several countries. The recently discoveredpopulation of the Hautes Fagnes (Belgium) seems to be in full expansion. This particular case constitutesa unique opportunity to understand the dynamics of a founder population over a vast area. We used siteoccupancymodelling to assess the distribution of this discrete species in this vast peatland habitat bydetermining the probability of use of 52 sample areas spread over nearly 5 000 ha. The population size wasestimated within six sample areas through capture-recapture method. The total population size was thenextrapolated by quantifying the surface of favourable habitats on the perimeter currently occupied. Ourresults indicate that the occupation area covers at present just over 800 ha, of which around 450 ha are suitable for breeding. Densities are high, counting between six and 14 individuals per hectare dependingon the locality. The population size is estimated between 3 000 and 6 000 individuals. Our results suggestthat less than 10% of the potentially suitable areas within the nature reserve are currently occupied. Thepopulation could therefore contain several tens of thousands of individuals in the future. Our results alsoshow that the probability of using the sites is negatively influenced by the tree cover, even when theseare in small numbers. This demonstrates the importance of maintaining open areas that are crucial forthe breeding process. Additionally, the probability of occupancy is positively correlated to the abundanceof ericaceous plants (Calluna vulgaris, Erica tetralix, Vaccinium myrtillus and V. uliginosum), sphagnum(Sphagnum sp.) and polytrics moss (Polytrichum sp.). On the other hand, the cover of Molinia caerulea isnegatively correlated to the occupancy probability. The Adder can therefore be considered as a goodindicator of the conservation status of heathlands. ; La Vipère péliade (Vipera berus) étant une ...