Two well-preserved specimens of a “cetothere” from the Lower Miocene Awa and Mizunami Groups, Japan, are described as Isanacetus laticephalus, gen. et sp. nov. It is distinguished by the following combination of characters: fossa for the stapedial muscle elongated anteroposteriorly, with a fully ossified floor; anteroposteriorly broad supraorbital process of frontal, with the orbital margin concave in dorsal view; posterior thrust of the medial rostral elements reaches the level of the center of the orbit; apex of the occipital shield reaches beyond the level of the anterior end of the zygomatic process of the squamosal; nasal slender, elongate, and partly located anterior to level of the preorbital angle of the supraorbital process of the frontal; and zygomatic process of the squamosal slender, elongate, and directed forward and slightly outward.

A parsimony analysis of 16 cetaceans and 77 characters failed to identify any synapomorphies for the “Cetotheriidae.” The analysis supports the following hypotheses: “Cetotheriidae” as commonly used is a paraphyletic grade; “cetotheres” are more closely related to Balaenopteridae Eschrichtiidae than they are to Balaenidae; and “cetotheres” form two subgroups, one which includes Cetotherium and another which includes Isanacetus, Parietobalaena, and Aglaocetus. The latter subgroup is more closely related to Balaenopteridae Eschrichtiidae, but is also paraphyletic.