The uniformity of bumble bee morphology has made it difficult to reconstruct phylogenetic relationships. Although male genitalia have been reasonably useful for grouping species into subgenera, resolution among species groups has been highly incomplete. DNA sequence data for 220 of the known 250 species from four gene fragments (16S, EF-1a, LW Rh and arginine kinase), analyzed within a mixed-model Bayesian framework, has resulted in a well resolved phylogeny of the bumble bees. This has allowed us to estimate the age of the major splits within the genus using gene by gene estimates of relative rates of evolution, calibrated with dates from bumble bee fossils. Optimization of the geographic locations of the taxa onto the tree using dispersal/vicariance analysis reveals the ancestral distributions of the major groups.