Marine Rhodobacterales are recognized as a widespread, abundant, and metabolically versatile bacterial group in the world's oceans. They also show a nearly universal conservation of the genes for production of gene transfer agents (GTAs), virus-like particles that mediate genetic exchange between cells. It is not yet clear what factors determine the distribution of the various taxonomic subgroups of this order. To address this question, we analyzed the Rhodobacterales communities in 10 seawater samples from northern Baffin Bay collected during September 2008. A conserved gene from the GTA gene cluster was used to characterize the Rhodobacterales community structure. A total of 320 clones from 10 clone libraries were sequenced, and 22 operational taxonomic units representing putative species and 13 clusters representing putative genera were identified. A cluster related to Octadecabacter comprised 59% of total clones from the northern Baffin Bay. Phylogenetic analysis of the clones showed that the Rhodobacterales communities had distinct compositions in the different water masses that were sampled. A change in community structure related to depth was also observed. Therefore, in northern Baffin Bay where two ocean currents meet and mix, the Rhodobacterales community structures were primarily determined by water mass and depth.