The otolith (sagitta, astericus, and lapillus) of the channel catfish (Ictalurus punctatus), the walking catfish (Clarias fuscus) and the loricated catfish (Hypostomus sp.) were examined for calcium carbonate polymorphism and crystalline morphology. Calcium carbonate polymorphisms were determined by Raman spectroscopy, and crystalline structure was observed directly using scanning electron microscopy. In the three catfish species, all of the otoliths were aragonitic, but shoed a trend towards decreasing crystal size from I. punctatus, through C. fuscus to Hypostomus sp. Combinations of twinning and selective inhibition of the growth of certain crystal faces, have resulted in crystalline habits ranging from pseudo-cuboidal to amorphous. In addition to catfish otoliths, the otolith (sagitta) of one of the most primitive Ostariophysine fish, Gonorhynchus gonorhynchus, was described. It is proposed that the variation in otolith shape can be regarded as allometric, resulting from the effects of species-specific regulatory genes on order-specific structural genes.