Both sexes of Andromastax muricatus, new genus, new species (Copepoda: Harpacticoida: Aegisthidae), are described from a hydrothermal vent on the Galapagos Rift in the eastern Pacific. This epibenthic species represents the most primitive aegisthid known to date, displaying several plesiomorphic character states in the cephalosomic appendages and in P5 and P6. The new species shares some characters with the imperfectly described Aegisthus spinulosus, which is removed from Aegisthus and placed here as a species incertae sedis in the Aegisthidae. The basic setal pattern on the caudal rami in the Aegisthidae is reconsidered and the different degrees of atrophy of the mouthparts in the nonfeeding males are discussed. It is postulated that the family evolved from an epi- or hyperbenthic ancestral stock and that the colonization of the open pelagic environment by Aegisthus is secondary.