Over the past decade, large populations of invasive quagga mussels (Dreissena rostriformis bugensis) have become established in reservoirs along the lower Colorado River in the southwestern United States. We were interested in whether genetic patterns among these populations have changed during continuing expansion and establishment, and how current genetic patterns in reservoirs along the lower Colorado River would compare with other invasive D. rostriformis bugensis populations. We examined the neutral genetic structure among six populations from different reservoirs along the lower Colorado River. We genotyped individual quagga mussels at 10 microsatellite DNA loci to analyze patterns of genetic diversity and population structure. As in past studies of some of these same populations, overall genetic divergence among populations was negligible and no single reservoir population was significantly differentiated from the overall group. Some populations did exhibit significant, if slight, pairwise genetic differentiation, and there was a moderate pattern of isolation-by-distance. Studies of microsatellite DNA-based population genetic patterns in invasive D. rostriformis bugensis in other parts of the world have been limited to recently established populations and likewise show similar absences of strong genetic structuring.