Xiphiorhynchus kimblalocki, a new species of extinct billfish from the Eocene of Mississippi, is described. This is the first record of Xiphiorhynchus outside of western Europe, and the material consists of a well-preserved rostrum, three partial vertebrae and two fin spine fragments. Xiphiorhynchus kimblalocki is compared with other living and extinct billfish and appears to be intermediate in morphology between the Xiphiidae and Istiophoridae. Various genera of fossil billfish are critically discussed and we suggest that the Blochiidae, Paleoihynchidae, and the "Cylindracanthus-group" should be placed in Xiphioidei Incertae sedis until better evidence indicates that they are billfish. We speculate that Xiphiorhynchus is an extinct offshoot from an unknown pre-Eocene common ancestor between Xiphiidae and Istiophoridae and is closer to the Istiophoridae than to the Xiphiidae. We also agree with earlier workers that the lineages of the Xiphiidae and Istiophoridae run back separately into basal Eocene times and that any common ancestry to each other and to the scombroids must have been prior to the Eocene and may have extended into the Cretaceous.