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To understand how allopatric speciation proceeds, we need information on barriers to gene flow, their antiquity, and their efficacy. For marine organisms with planktonic larvae, much of this information can only be obtained through the determination of divergence between populations. We evaluated the importance of ocean barriers by studying the mitochondrial DNA phylogeography of Tripneustes, a pantropical genus of shallow water sea urchin. A region of cytochrome oxidase I (COI) was sequenced in 187 individuals from locations around the globe. The COI phylogeny agreed with a previously published phylogeny of bindin that barriers important to the evolution of Tripneustes are: (1) the cold water upwelling close to the tip of South Africa, (2) the Isthmus of Panama, (3) the long stretch of deep water separating the eastern from the western Atlantic, and (4) the freshwater plume of the Orinoco and the Amazon …