A new genus and species of protocetid cetacean, Carolinacetus gingerichi, is described from a partial skull, the posterior portion of both dentaries, 13 vertebrae, and elements of 15 ribs found in the Cross Member of the late middle Eocene Tupelo Bay Formation in Berkeley County, South Carolina. That formation is also defined and named in this paper. Although the holotype skull of Carolinacetus is fragmentary, it includes the best preserved petrosal of any described specimen from the archaeocete families of Pakicetidae, Ambulocetidae, Remingtonocetidae, and Protocetidae. The phylogenetic relationships of Carolinacetus were determined by a cladistic analysis of a dataset that includes 16 cetacean taxa and 5 outgroups scored for 107 morphological characters. Carolinacetus was found to be the basalmost cetacean known from North America, and the most conspicuous character supporting this position is the external, bony nares being anterior to P1. Other noteworthy findings are that Georgiacetus is more basal than Babiacetus and that Remingtonocetidae occupies a branch between Pakicetidae and Protocetidae. Based on our phylogenetic analysis, the genus Gaviacetus is removed from the Basilosauridae and restored to the Protocetidae. Possible dispersal routes of protocetids from the Old World to the New World are discussed, and a route westward along the southern coast of Greenland is considered to be the most likely avenue of protocetid migration to North America.