A new, relatively complete and well-preserved mandible and dentition of the mesonychian Ankalagon saurognathus from the early Paleocene (Torrejonian) of the San Juan Basin, New Mexico, provides evidence of sexual dimorphism in dentary depth and canine size in Mesonychidae. The new specimen has a particularly well-preserved postcanine dentition, and m1 and m2 differ little in morphology or size from those in the holotype. The p3–p4 and m3 in the new specimen are smaller in length and width than in the holotype, but the size differences fall within the intraspecific range of variation exhibited in extant carnivorans. The holotype differs from the new specimen in having a canine that is approximately 1.5 times greater in length and width, as well as a dentary that is substantially deeper and larger. It has been suggested that two other mesonychians, Hapalodectes and Dissacus, also exhibit sexual dimorphism but evidence for this remains equivocal. We discuss the phylogenetic and behavioral implications of this character.