In 1999 and 2004, we published reports on how the introduction of 20 males into a severely inbred and isolated population of Adders (Vipera berus) halted its decline towards extinction. The introduction significantly enhanced the population's genetic variability, which resulted in a dramatic increase in offspring viability and a rapid increase in numbers. Unfortunately, recently a new and unprecedented development is threatening the population's future survival. In 2004 permission was granted by the Swedish Nature Conservation Agency of the County Administrative Board to build a house and an adjacent 1 m tall brick wall, right across the habitat occupied by the Adders. The construction of the house and brick wall in 2006 has prevented the majority of the snakes from undertaking their annual migration within the study area, resulting in an extirpation > 75% of the Adders. This reduction seriously impedes the future survival of this unique population.