Xiphactinus audax is the largest macropredatory osteichthyan ever known (Everhart et al., 2010). Some of the largest specimens exceed 5 m in total length, although the discovery of a few large, isolated teeth suggests that this teleost could reach even larger body sizes (Vavrek et al., 2016, and references therein). Fossil remains of this species have only been reported from the Upper Cretaceous of North America, across the Western Interior Basin (Schwimmer et al., 1997; Vavrek et al., 2016). The discovery of several virtually complete individuals in this area has provided valuable information about the anatomy, the dimensions, and the ecology of this species (Cope, 1872; Bardack, 1965). Xiphactinus audax displayed a tarpon-like body plan with a semilunate caudal fin and large caniniform teeth (Hay, 1898; Bardack, 1965; Carrillo- Briceño et al., 2012), suggestive of a highly active predatory lifestyle (Cavin et al., 2013). In fact, the emblematic 'fish-within-afish' specimen, containing an entire 2-m-long Gillicus arcuatus, provides evidence of the ability of X. audax to prey upon large, rapidly swimming fishes (Bardack, 1965). Predation entails a high energetic demand (Brown and Kotler, 2004). Ferrón et al. (2017) recently proposed that, because mass-specific metabolic rate decreases with increasing body size, highly active lifestyles (such as macropredation) cannot be maintained by an ectothermic metabolism over a specific body size. From this perspective, they argued that the punctual evolution of gigantism among macropredators was closely linked to metabolic-level shifts promoted by various factors (i.e., endothermy, highly efficient respiratory systems, warm temperatures, and high oxygen levels) and suggested that several extinct aquatic macropredators, including Xiphactinus, could have been meso- or endotherms on the basis of their body size and life history. Later, Ferrón (2017) established a useful methodology to assess the swimming energetics of extinct aquatic organisms, which can be used to interpret their metabolic levels and thermoregulatory strategies. This study presents an equivalent methodology developed for assessing locomotion energetics in extinct osteichthyans and provides evidence of endothermy in X. audax.