Snailfishes of the genus Careproctus deposit egg masses inside the gill cavity of lithodid crabs. Previous attempts to identify the species depositing the eggs have been inconclusive because of the difficulties in identifying snailfish eggs and larvae. For this study, samples were collected from egg masses found in crabs in the commercial fishery and during benthic trawl survey work. Egg masses were found in Lithodes aequispinus (Golden King Crab) and, for the first time, in L. couesi (Scarlet King Crab). Mitochondrial DNA from the cytochrome c oxidase subunit I (COI) region was amplified from embryos and sequenced. Comparison of COI sequences from eggs to COI sequences from positively identified adult voucher specimens yielded identifications of 75 egg masses to species. A total of 38 egg masses were identified as Careproctus melanurus; 29 as C. colletti; seven as C. furcellus; and one as C. simus. Each egg mass contained eggs from only one species, and there was no indication of multiple maternal parentage. These results are the first positive identification of the snailfish species responsible for depositing egg masses in lithodid crabs. Identification of species involved may be useful in understanding the nature of this unique relationship between snailfishes and crabs.