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Small populations with unusual characteristics subject to extreme conditions provide opportunities for exploring adaptability in the face of environmental changes. Two sets of data have been examined to determine how unusual is the population of Hipparchia semele on the Great Orme's Head, North Wales, compared with other sites in the UK. The population on the Great Orme is shown to have unique features, including significantly reduced wing expanse and wing ocellation and extreme flight period characteristics. Analyses of flight period data from the UK Butterfly Monitoring Scheme (UKBMS) using over a 100 sites reveals that, although the Great Orme population is one of a number of sites from the Channel Islands to northern Scotland with an early mean flight period, it has by far the earliest flight period and longest flight period of all populationsthe latter raising the mean flight period date. Furthermore the unique characteristics of H. semele on the Orme may well be underestimated, inasmuch as sampling of individuals for the phenotype study is incomplete, including only the area along the North Wales coast into Cheshire, while the UKBMS transect is restricted to the south-west portion of the headland. Unique populations are often accorded focused conservation effort; especially potential flagship species in decline as in the case of British H. semele. As the Great Orme population presents a rare opportunity for studying adaptations in an extreme local environment, particularly considering current projections for climate changes, we advocate further research and attention being given to this unusual population.