Drosophila mojavensis and Drosophila arizonae, a pair of sibling species endemic to North America, constitute an important model system to study ecological genetics and the evolution of reproductive isolation. This species pair can produce fertile hybrids in some crosses and are sympatric in a large part of their ranges. Despite the potential for hybridization in nature, however, evidence of introgression has not been rigorously sought. Further, the evolutionary relationships within and among the geographically distant populations of the two species have not been characterized in detail using high-resolution molecular studies. Both species have six chromosomes: five large acrocentrics and one 'dot' chromosome. Fixed inversion differences between the species exist in three chromosomes (X, 2 and 3) while three are colinear (4, 5 and 6), suggesting that were introgression to occur, it would be most likely in the colinear chromosomes. We utilized nucleotide sequence variation at multiple loci on five chromosomes to test for evidence of introgression, and to test various scenarios for the evolutionary relationships of these two species and their populations. While we do not find evidence of recent introgression, loci in the colinear chromosomes appear to have participated in exchange in the past. We also found considerable population structure within both species. The level of differentiation discovered among D. arizonae populations was unexpectedly high and suggests that its populations, as well as those of D. mojavensis, may be themselves undergoing incipient speciation and merit further attention.