In this study, for the first time in Iran, the karyotype of bigmouth Lotak, Cyprinion macrostomum Heckel, 1843, was investigated through examining metaphase chromosomes of seven fish with mean weight 30±5g caught by electrofishing from Godarkhosh River in Ilam Province. To stimulate cell divisions, fish were injected intraperitoneally two times by phytohemagglutinin (PHA). The cell divisions were arrested in metaphase stage by intraperitoneal injection of colchicine. Well-separated cells were obtained from kidney and gill filament and chromosome spreads were prepared and stained with giemsa. Karyotype was obtained as 2n=50. The karyotype consisted of 5 metacentric, 12 submetacentric and 8 telocentric chromosome pairs. Centromeric index, arm ratio and Fundamental Number (FN) were determined as 0-50, 1-∞, and 84, respectively.