Recent studies on the microbial flora of the honeybee gut have revealed an apparently highly specific community of resident bacteria that might play a role in immune defence and food preservation for their hosts. However, at present, very little is known about the diversity and ecology of bacteria occurring in non-domesticated bees like bumblebees, which are of similar importance as honeybees for the pollination of agricultural and wild flowers. To fill this gap in knowledge, we examined six of the most common bumblebee species in Central Europe from three locations in Germany and Switzerland for their bacterial communities. We used a culture-independent molecular approach based on sequencing the 16S rRNA gene from a selection of individuals and examining a larger number of samples by terminal restriction fragment length polymorphism profiles. The gut flora was dominated by very few and mostly undescribed groups of bacteria belonging to the Proteobacteria, Bacteroidetes, Firmicutes and Actinobacteria. This core set of bacteria was present in all of the examined bumblebee species. These bacteria are similar to, but distinct from, bacteria previously described from the honeybee gut. Significant differences were observed between the communities of bacteria in the different bumblebee species; the effect of sampling location was less strong. A novel group of Betaproteobacteria additionally shows evidence for host species-specific genotypes. The gut flora of bumblebees therefore is apparently composed of relatively few highly specialized bacteria, indicating a strong interaction and possibly important functions with their hosts.