We describe the odontocete, Ashleycetus planicapitis, gen. et sp. nov., based on a partial skull that was collected from the upper Rupelian (lower Oligocene) Ashley Formation near Charleston, South Carolina, and place it in the Ashleycetidae, fam. nov. Overall, the cranial morphology of this new species is plesiomorphic; it has an elongate, tabular intertemporal region, external nares well anterior to the antorbital notches, and anteromedially oriented lateral margins of the supraorbital processes. Xenorophus sloanii and Xenorophidae are rediagnosed, and the latter is also redefined as an apomorphy-based clade. This new definition should lead to greater stability because it excludes Archaeodelphis patrius, whose phylogenetic relationships have proven to be problematic. The holotype skull of the archaic odontocete Mirocetus riabinini, from probable upper Rupelian sediments of the lower Maikop beds of Azerbaijan, is redescribed, newly figured, and placed in the Mirocetidae, fam. nov. Previously regarded as an archaeocete, Mirocetus is here shown to be an odontocete. We added Ashleycetus, Albertocetus, and Mirocetus to a recently published supermatrix of morphological and molecular data. Although Ashleycetus, Archaeodelphis, and Mirocetus consistently occupy basal branch(es) within Odontoceti, their exact positions are sensitive to the value of a constant used in implied weighting for cladistics analysis. Three of the characters we find to be odontocete synapomorphies are closely associated with soft tissue structures implicated in the production and transmission of high-frequency sounds during echolocation. Detailed dissections of extant odontocetes are required to determine if these features can be considered evidence of echolocation in extinct taxa.