1. Maculinea alcon, a myrmecophilic, stenotopic lycaenid butterfly is restricted to wet heathlands, bogs, and nutrient-poor hay meadows. Due to intensification of agriculture and decrease of extensively grazed meadows, many suitable habitats have disappeared and the remaining ones are highly fragmented and deteriorated. 2. Historical distribution data and a comparison with the present occupation of patches show the decline of this critically endangered butterfly in north-west Germany. Most of the populations in north-west Germany are small and often geographically isolated. 3. In summer 2002, two-thirds of 77 investigated potential patches were unoccupied as a result of unsuitable habitat structure and habitat fragmentation. 4. Several habitat parameters were highly significantly correlated with the presence of M. alcon, in particular the distribution pattern of the host plant Gentiana pneumonanthe. Furthermore, butterflies were absent from many patches with an incidence probability below 50% with respect to patch size and isolation. 5. In the nature reserve Lüneburger Heide, part of the study area, M. alcon populations have been observed since 1995. Typical turnover of local populations could be detected during these years. Extinctions and re-colonisations have stabilised the presence of this species in a metapopulation in this nature reserve. 6. These data show the importance of different factors on different spatial levels influencing the presence of this endangered butterfly. © 2007 The Royal Entomological Society.