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1. The common dolphins (genus Delphinus) have one of most problematic taxonomies and complex distribution patterns of all cetaceans. Although the taxonomy and the distribution seem to have been clarified somewhat in the eastern North Pacific and Indo-Pacific Oceans, many questions remain in the Southwestern Atlantic Ocean (SWA). We review the biogeography of Delphinus in the SWA. 2. We reviewed data from strandings, incidental catches and sightings since 1922. Systematic surveys were conducted in five major areas. Twenty-one natural history collections were examined, and 135 skulls were measured. 3. A total of 184 records of common dolphins were compiled. Delphinus apparently occurs in three stocks in the SWA: one located in northern Brazil and two from southeastern Brazil (∼22°S) to central Argentina (∼42°S). Two distinct patterns in habitat use were observed by depth: in southeastern Brazil, sightings were restricted to coastal waters with water depths ranging from 18m to 70m. On the other hand, in the area that extends from southern Brazil to Central Argentina (from 28°S to 42°S), sightings were recorded in deeper waters, ranging from 71m to 1435m, with the exception of occasional coastal sightings. The cranial analyses demonstrated that both short-beaked common dolphins Delphinus delphis and long-beaked common dolphins Dephinus capensis occur in the SWA. 4. In the SWA, Delphinus seems to occur near areas of high productivity. One stock is associated with the productive waters discharged by the Amazon River and possibily with the coastal upwelling system off the coast of Venezuela, while the other stocks are associated with the Cabo Frio upwelling system and the Subtropical Convergence. Our results indicate that the current taxonomy does not adequately reflect the amount of variation within the genus in the world. [ABSTRACT FROM AUTHOR]