Ontogenetic development of ornamentation and whorl geometry of the Cretaceous ammonoids Parajaubertella kawakitana and P. zizoh are studied based on well-preserved specimens collected from the lower Cenomanian in the Horokanai area, Hokkaido, Japan. Our results indicate that their comparably sized immature stages share identical ornamentation and shell morphology, while the size of their adult shells is distinctly bimodal. They also share the same stratigraphic ranges in the lower Cenomanian and have overlapping geographic distributions in Northwest Pacific region, and lastly, they co-occur in the same concretions. This evidence strongly suggests that the two taxa should be considered as dimorphs, microconch and macroconch of a single species, which is herein described as P. kawakitana.