Asteriscus, sagitta and lapillus otoliths of freshwater fishes (Cyprinids) from the Friuli Plain were mineralogically and chemically analyzed. All the analyzed fish showed that sagitta and lapillus consist of aragonite whereas asteriscus is made of vaterite. Chemical analysis showed that different Na, K, Mg and Sr concentrations are highly correlated with the mineralogy of the otoliths. Moreover there was a different trend of accumulation between vateritic and aragonitic otoliths. In particular it has been found that Mg is mainly present in vaterite otolith (asteriscus), while Sr is mainly present in aragonite otolith (lapillus). This fact was confirmed by statistical analyses. Our hypothesis is that this unusual high content of Mg could be related to the organic matrix present in asteriscus otolith, and, consequently, that the different organic matrix responsible for the different calcium carbonate polymorph is strictly correlated with the concentrations of at least some microelements.