A 395 kg shortfin mako, Isurus oxyrinchus (Lamnidae) landed at Montauk, New York in June of 1992 was found to contain two dead, but otherwise healthy appearing pugnose eels, Simenchelys parasiticus (Synaphobranchidae) within the lumen of its heart. The path along which the eels made their way into the shark's heart was not found. Histological examination of the shark's heart revealed the presence of medial hyperplasia and hypertrophy of small arterioles, multifocal arteriosclerosis, and relatively high numbers of capillaries per unit area. These phenomena were not observed in similar sections taken from hearts of six uninfected mako sharks. The stomachs of both eels were filled with blood, suggesting that they had been within the shark at least long enough to feed. Consideration of this new record along with the literature supports a trophic designation of facultative endoparasite for the species Simenchelys parasiticus.