The genus Cyprinion , of the family Cyprinidae, is distributed only in Asia . There are nine species in this genus, amongst which, five are reported from Iran . The species C. kais , C. macrostomum and C. tenuiradius were reported from Karkheh and Karun River basins . Based on various reports, their taxonomy is uncertain. In this study, 282 specimens of C. macrostomum and 24 specimens of C. kais from Karkheh River basin (Do-Ab, Gamasiab, Saimare, Kashkan, Chardavol and Karkheh Rivers) and Godarkhosh River in Ilam province were caught, then fixed, freezed or traorted alive to Ichthyological Laboratory, Department of Natural Resources, Isfahan University of Technology. To study the taxonomy of the specimens, morphological, osteological and karyological traits were used. Parametric and non-parametric methods for comparison of means and Factor Analysis for separation of groups were used. The last dorsal fin spine in C. kais was longer than in C. macrostomum (the SD ± average of dorsal fine height relative to standard length were 0.19 ± 0.02 and 0.18 ± 0.01, respectively). The posterior spinules of last dorsal spine in C. macrostomum were equal in size, but different in C. kais and its tip was softer and more flexible than in C. macrostomum . The mouth form in C. kais was narrow and highly arched, but in C. macrostomum it was wide and less arched (the SD ± average of mouth radius relative to mouth width were 0.39 ± 0.06 and 0.28 ± 0.04, respectively). The joint between the two dentaries in C. kais were weaker than in C. macrostomum and the maxillary and premaxillary in C. kais were more arched than in C. macrostomum . The diploid number in both taxa was 2n = 50; in C. kais included eight metacentric, 10 submetacentrics and seven acro and telocentric chromosome pairs and FN = 86; and in C. macrostomum included four metacentric, 13 submetacentric and eight telocentric chromosome pairs and FN = 84. There were no significant differences between the biometrical characteristics in the two taxa in other characters (p 0.05). Based on the observed differences in chromosomal formulae, mouth characteristics, and the last dorsal fin spine, the two taxa, C. kais and C. macrostomum are distinguished at least as suecies. Although the two taxa could also be distinguishable at species level, they are not completely separated as good species, possibly, due to their recent speciation.
