Margolisius abditus n. gen., n. sp. (Copepoda: Lernaeopodidae) is described from female specimens collected from gill lamellae of a remora, Remora remora (L.), captured in the Gulf of California near Punta Arena, Baja California. Comparison of this species with 13 lernaeopodid genera with which it shares (a) absence of posterior trunk processes; (b) presence of a relatively long cylindrical cephalothorax, usually reflected along the dorsal surface of the trunk; and (c) a marine habitat showed that it cannot be placed in any of them, thus necessitating establishment of a new genus. Margolisius abditus n. sp. is the smallest known lernaeopodid species, its ovigerous females measuring about 0.5 mm in total length.