This study of more than 500 specimens recognises two cetomimid subfamilies (one newly described), nine genera (four newly described) and about 35 species (four newly described). Characters of the gill arches, head laterosensory canals, lateral line scales, cavernous tissue, anal lappets and subpectoral organ (the last three of unknown function) are used to distinguish taxa. A cladistic analysis of 39 characters utilised Rondeletia and Barbourisia as outgroups to polarise character states within the family and the beryciform families to polarise family characters. Three synapomorphies, the presence of gill rakers in some form other than elongate and flattened and the absence of pelvic fins and pleural ribs, support the monophyly of the family. The monotypic Procetichthyinae, defined by four autapomorphies, is the primitive sister group of all other cetomimids. It retains such pleisiomorphic features as a fully developed nasal organ, a fully developed eye with lens, a pseudobranch and 19 principal caudal rays. Ditropichthys is the primitive sister group of the remaining seven genera, with the next line including the related Cetichthys and Notocetichthys. The relationships of the remaining five genera are equivocal, except that Cetomimus and Gyrinomimus are sister taxa defined by lateral line scale shape. A working hypothesis of relationships is presented that places Danacetichthys and Cetostoma as sister groups of the remaining three genera. Multistate characters of gill raker-tooth plate shape and extent of the fourth gill slit support the hypothesis. Cetostoma and Rhamphocetichthys are highly derivative forms, with each monotypic genus defined by three or four autapomorphies. The genera Gyrinomimus and Cetomimus each have more than ten species, which will be reviewed in later papers. The other genera each have one or two species that are fully detailed here. All examined specimens with recognisable gonads are females, confirmed by histology of eight specimens representing four genera. Egg sizes of at least 2.0, 1.3 and 0.6 mm diameter are attained by the genera Procetichthys, Gyrinomimus and Cetostoma respectively. The vast majority of examined specimens had eggs 0.1 mm diameter or less. The few specimens with larger eggs had bimodal egg sizes. Males and individuals less than 25 mm are unknown. Maximum size is at least 390 mm in one species of Gyrinomimus, while Ditropichthys and Cetostoma apparently do not exceed 140 mm and 250 mm, respectively. Crustaceans are the primary food of whalefishes. The family is distributed in all oceans, from 52°N to 72°S. At the species level, two distribution patterns are apparent. The two commonest species, Cetostoma regani and Ditropichthys storeri, have cosmopolitan distributions between 50°N and 40°S. The two most frequently captured species of Gyrinomimus are restricted to the north Pacific between 39° and 52°N and circumglobally in the Southern Ocean between 32° and 72°S. There are too few collections of the other species to ascertain distributional limits, but some have been taken in all three oceans and others only in a part of one ocean. The centres of vertical distribution for all species are below 1000 m. Only smaller specimens of C. regani and D. storeri have been taken above 1000 m, where some nocturnal vertical migration is indicated. Closing net captures confirm large specimens of both species live between 1200 and 1500 m. Four separate closing net captures of Cetichthys parini between 2700 and 3200 m demonstrate this to be the deepest living known whalefish. At least some cetomimids are more abundant than previously thought, with 55% of the ISH midwater trawls in the Atlantic fishing to 1800 m or deeper catching whalefishes. Up to 11 specimens and six different species of whalefishes were taken in a single deep trawl. With about 35 species, cetomimids are second only to the anglerfish family Oneirodidae as the most speciose bathypelagic fish family and may be the most abundant below 1800 m. As four of the eight species considered in this paper are represented by less than five specimens, it seems probable that additional species of cetomimids will be captured.