Copepods are common parasites of marine fishes, but little information is available on the biology of species that parasitize mesopelagic fishes in oceanic waters. In this study, we report the finding of large numbers of Sarcotretes spp. (n = 2,340) in dietary samples of king penguins collected at Crozet and the Falklands Islands. Analysis of penguin food indicates that S. scopeli Jungersen parasitizes myctophid fishes, Protomyctophum tenisoni (Norman), in the southern Indian Ocean and P. choriodon Hulley in the southern Atlantic. It suggests that the much rarer S. eristaliformis (Brian) also parasitizes myctophids, but the host species of that copepod remains to be determined. The new data add significant information concerning the hosts and distribution of Sarcotretes spp. in the Southern Ocean and emphasize the usefulness of ichthyophagous predators in revealing valuable information on the biology of organisms that parasitize their prey.