One hundred and twenty-one species of deep sea “cephalaspidean” heterobranchs belonging to the genera Acteon, Crenilabium, Obrussena, Rictaxis, Japonacteon, Maxacteon, Bullina, Diaphana, Toledonia, Cylichna, Scaphander, Sabatia, Roxania, Cylichnium, Acteocina, Truncacteocina, Philine, Retusa, Pyrunculus, Volvulella, Relichna, Micratys, Gastropteron, Aglaja and Philinopsis are reported from the tropical southwest Pacific. Thirty-nine of these species are new: Acteon ionfasciatus, Acteon chrystomatus, Rictaxis sanguinea, Japonacteon longissimus, “Acteon” editus, “Acteon” buccinus, “Acteon” ringiculoides, “Acteon” boteroi, “Acteon” loyautensis, “Acteon” rhektos, “Acteon” profundus,“Acteon” osexiguus, “Acteon” aphyodes, “Acteon” herosae, “Acteon” comptus, “Acteon” chauliodous, “Acteon” cohibilis, Bullina rubropunctata, Toledonia neocaledonica, Toledonia epongensis, Cylichna tanyumphalos, Cylichna grovesi, Sabatia pyriformis, Roxania smithae, Cylichnium mucronatum, Cylichnium nanum, Acteocina lata, Philine habei, Philine babai, Philine abyssicola, Retusa diaphana, Retusa insolita, Retusa lenis, Retusa abyssicola, Retusa trunca, Volvulella onoae, Volvulella multistriata, Relichna hadra and Micratys wareni. A previously described species, Acteon aequatorialis, is included in the new genus Bathyacteon. Three species are assigned provisionally to already described species until more material becomes available: Acteon cf. nakayamai, Maxacteon cf. kawamurai, “Acteon” laetus. Thirty-eight species remain unnamed because of the absence of adequate information, but the shells are illustrated. Most species are described based on conchological data. Fourteen species of Acteonidae and two of Retusidae are provisionally assigned to the artificial taxa“Acteon” and “Retusidae” until anatomical data become available. The present collecting effort in the southwest Pacific has produced large numbers of previously undocumented species. The largest number of species was found in the area comprising the Coral Sea, New Caledonia, Vanuatu, Fiji, Tonga and Wallis and Futuna, which is probably a consequence of a greater collecting effort. The list of species reflects a high degree of endemism in the deep sea fauna from the southwest Pacific. Only a few widespread Indo-Pacific species have been found in the deep sea. It also appears that there is some sort of isolation between the Coral Sea, New Caledonia, Vanuatu, Fiji, Tonga and Wallis and Futuna region and the Philippines and Indonesia region, which is reflected in the small number of species shared between these two areas. Most species of “cephalaspidean” heterobranchs studied here have broad bathymetric ranges compared to other groups of opisthobranchs, which may be a result of a higher ecological adaptability of this group, or may be an artifact caused by transport of empty shells. When only specimens collected alive are considered, the bathymetric ranges of most species are considerably narrower. Most species studied are exclusively found in the deep sea, but a small number of shallow water species have been recorded here for the first time in deep waters. When the ranges of empty shells are examined there appears to be a turnover of “cephalaspidean” heterobranch species at about 1000-1200 m depth and a blurry transition between shallow waters and the deep sea. When only specimens collected alive are considered, there is a sharp boundary at about 200 m that clearly separates the shallow water and the deep sea faunas. “Cephalaspidean” heterobranch species are more common relative to other groups of opisthobranchs in deep waters than in shallow waters, but this result may be an artefact caused by the collecting techniques.