Aspen (Populus tremuloides) is a temperate North American tree species with a geographical distribution more extensive than any other tree species on the continent. Because it is economically important for pulp and paper industries and ecologically important for its role as a foundation species in forest ecosystems, the decline of aspen in large portions of its range is of serious concern. The availability and annotation of the black cottonwood (Populus trichocarpa) genome enables a range of high throughput sequencing approaches that can be used to understand rangewide patterns of genetic variation, adaptation, and responses to environmental challenges in other Populus species, including aspen. Gene expression studies are particularly useful for understanding the molecular basis of ecological responses, but are limited by the availability of transcriptome data. We explored the aspen transcriptome through the use of high-throughput sequencing with two main goals: (1) characterization of the expressed portion of the P. tremuloides genome in leaves and (2) assessment of variation in gene expression among genets collected from distinct latitudes but reared in a common garden. We also report a large single nucleotide polymorphism dataset that provides the groundwork for future studies of aspen evolution and ecology, and we identify a set of differentially expressed genes across individuals and population boundaries for the leaf transcriptome of P. tremuloides.