Levels of divergence in the mitochondrial cytochrome c oxidase subunit I gene were assessed from geographically widespread populations of Bugula dentata Lamouroux. An average pairwise distance of 21% (Kimura-2P corrected) was found between colonies from the eastern Australian seaboard, Albany in south west Australia, and Oahu in Hawaii. Assuming COI mutation rates are comparable with other organisms, cladogenisis leading to these taxa would have occurred in the Miocene. Morphological differences between specimens from eastern Australia arid Hawaii are consistent with regional taxonomic differences identified previously in the literature, while phylogenetic division between the southern Australian specimens has not been diagnosed taxonomically. Examination of population-level differences within south eastern Australia revealed a shallow phylogenetic structure consistent with a recent colonisation event by a small number of founders. Genetic differences between populations are structured (FST. = 1.0), suggesting small scale barriers to gene flow.