In a secluded corner of the Snowdonia National Park, an isolated population of the Silver-studded Blue butterfly Plebejus argus thrives. It is the only remaining northern colony occupying a wet peatland site, and is some miles from Silver-studded Blue localities on the Anglesey coast and the Great Orme. Althogh easy to confuse at first glance with the Common Blue Polyommatus icarus, the Silver-studded is extremely sedentary, highly gregarious and hard to miss when flying at mating; so it is surprising that it was over-looked for so long. Its story illuminates some important general conservation principles, and is a good illustration of the law of unintended consequences.