A new aulopiform, Ursichthys longiparietalis, gen. et sp. nov., preserved within and around the body cavity of a mosasaur, Mosasaurus missouriensis, is described from the Bearpaw Formation (upper Campanian, ca. 74.8 Ma) of Alberta, Canada (paleolatitude = 57°N). The partial skeleton consists of a partial skull associated with other isolated bones, including parasphenoid, hyomandibula, cleithrum, caudal peduncle centrum, scale, and many disarticulated fin rays. Many of those elements show evidence of bite damage considered to have been inflicted by the mosasaur. This taxon can be differentiated from other aulopiforms in having a dorsoventrally compressed neurocranium; short frontals; large, long, rectangular parietals that contact at the midline and are situated anterior to the supraoccipital; triangular epiotics, with the facet for the dorsal arm of posttemporal centered over the epiotic; hyomandibula head articulating anteriorly with only the pterotic and sphenotic; cleithrum with a long, narrow, straight vertical shaft; mesacoracoid present; and centra with two lateral, elongate fossa. Total length is estimated at 100 cm and age is estimated at 7 years old. We also emend the diagnosis for Apateodus to include more cranial characteristics. Among 37 known genera of Cretaceous aulopiforms, the diversity at the K-Pg boundary is probably underrepresented. Fish skeletons have rarely been recovered from the Bearpaw Formation in Alberta, likely due to a preservational bias. This report, however, shows promise for additional fish material preserved in mosasaur skeletons because they are relatively more commonly preserved than fish skeletons.