Several species of crabs from hydrothermal vent sites in the Pacific Ocean were found to be infested by small, symbiotic nemertean worms. Worms occurred on both male and female crabs, and were located in mucous sheaths adhering to the axillae between the limbs of males and females, the setae of the pleopods of females, and the sterna of infested male and female crabs. Only juvenile and regressed adult worms were observed, primarily because no ovigerous hosts were examined. Similar species of worms mature by eating eggs, then regress or die after host eclosion. Based on the size of the worms from the vent crabs, their habitus with their crustacean hosts, the presence of accessory stylet pouches, and the presence of a single stylet on a large basis (monostiliferous), we place the worms in the family Carcinonemertidae, within the genus Ovicides. Infestations were found on crabs from vent sites on the western Pacific back-arc basins, on the southern East Pacific Ridge, and on the Pacific-Antarctic Ridge, indicating a widespread distribution of the symbioses. This represents the first record of Carcinonemertidae from a deep-sea host, a new host family, Bythograeidae, for these symbionts, as well as the first record of parasitism on a deep-sea bythograeid crab.