The genetic and geographical variation, gene flow, and the historical biogeography of the eastern lubber grasshopper, Romalea microptera (=guttata) (Houttuyn), were examined by sequencing a 420-bp region of the mitochondrial cytochrome b gene. Individuals (168) were collected from 12 sites in the southern United States that covered most of the range reported for this species. These populations contained 49 mitochondrial DNA haplotypes. A high level of genetic diversity was observed in these populations (3.8%), most of which was due to variation within populations. The highest genetic variation was found in a northern Florida population (collected at St. Marks, FL), and the lowest was found in a southern Florida population (Copeland, FL). Estimates of historical and current population sizes suggest that most of the lubber populations drastically declined in size at some point in the past. In contrast to previous studies on several other species in this region, phylogenetic analyses (PAUP) and haplotype age phylogenies (PHYLIP) showed no major geographic structure. These observations suggest that the distribution of this species in the past may have been homogeneous, rather than the patchy distribution that is currently observed. Alternatively, this result may reflect the absence of long-term barriers for the dispersal of this species. Either of these might have contributed to the lack of a genetic structure divided geographically into east-west groupings that is seen in other species from this region.