A considerable number of species of lichen-forming fungi have wide geographical distributions, but studies of their genetic variability are minimal. ITS rDNA sequences of 32 populations of Parmelia saxatilis from five continents revealed two monophyletic groups. β-tublin gene sequences from a subset of nine collections supported these conclusions. While the number of collections sequenced is limited, one monophyletic group (the Atlantic Population. AtP) was recognized as occurring in Arctic and Antarctic regions and also included collections from more atlantic sites. Samples from more mesic environments in the Mediterranean region belonged to a second monophyletic group (the Mediterranean Population, MeP). In addition, four subgroups were distinguishable within the Atlantic Population. Norstictic and protocetraric acids are reported from the species for the first time, the norstictic acid only being found in the Atlantic Population. Living thalli from the Atlantic Population were provenance-tested; specimens transported from the UK to central Spain where the Mediterranean Population occurs showed adverse symptoms after six months. These results demonstrate that there can be substantial large-scale genotypic variability within widespread lichen phenospecies, something which has implications for comparative ecological, physiological, and air pollution sensitivity studies as well as for lichen conservation.