Abstract Although the allopatric model of speciation is widely accepted, it does not provide a satisfactory explanation for many evolutionary phenomena. Several alternative models exist, but they remain largely untested for vertebrate animals. In the present paper, a molecular phylogeny was used to test competing models of speciation in a seabird family, the Sulidae. A segment including 807 base pairs of the mitochondrial cytochrome bgene was sequenced from all extant sulid species, and phylogenetic methods were used to test model-specific predictions regarding tree topologies, distributions of sister taxa, timing of vicariant events, and comparative biology. Both the neighbor-joining and parsimony analyses placed sequences of gannets ( Morusspp.) and boobies of the genus Sulain separate, monophyletic lineages. Sequences of Cape ( M. capensis) and Australasian ( M. serrator) gannets clustered together, and the sequence of Abbott's booby ( Papasula abbotti) was basal to those of the gannets. Sequences of blue-footed ( S. nebouxii) and Peruvian ( S. variegata) boobies were sisters and formed a monophyletic group with the masked booby ( S. dactylatra). The red-footed booby ( S. sula) sequence was the most divergent of the Sulaboobies. All relationships received strong support from standard-error tests and bootstrap analysis. Substitution rates were similar to those suggested for mammals and suggested that most lineages arose within the last 3 million years. Lineage divergence events for which the mode of speciation could be deduced did not fit the predictions of either allopatric or sympatric models, but apparently involved either peripatric or parapatric processes.