Technical Abstract: The increasing evidence for population declines in bumble bee (Bombus) species worldwide has accelerated research efforts to explain losses in these important native pollinators. In North America, a number of once widespread Bombus species have suffered serious reductions in range and abundance, although other species remain healthy. To examine whether declining and stable species exhibit different levels of genetic diversity or population fragmentation, we genotyped a set of microsatellite markers from populations sampled across the geographic distributions of two declining and four stable Bombus species. Results show that populations of declining species generally have reduced levels of genetic diversity throughout their range compared to co-distributed stable species. Genetic diversity can be affected by overall range size and degree of isolation of local populations, which may confound comparisons among species in some cases. We find no evidence for consistent differences in gene flow among stable and declining species, with all species exhibiting strikingly weak genetic differentiation over large distances (e.g., >1,000 km). Populations on offshore islands and at high elevations experience relatively strong genetic drift, suggesting that some ecological conditions lead to genetic isolation in otherwise weakly differentiated species. Two species, B. bifarius and B. occidentalis, exhibit stronger genetic differentiation than the other four species, indicating more complex phylogeographic histories consistent with their broader geographic distributions in topologically heterogeneous regions of western North America. Screening of North American bumble bees for genetic diversity may prove to be a useful tool for rapidly identifying species of conservation concern. Development of conservation strategies allowing high intraspecific gene flow will help maintain cohesive populations that will help to prevent future declines in Bombus biodiversity.