he Campanian of the eastern Koppeh-Dagh Basin (NE Iran) is generally considered to be represented by the upper part of the Abderaz and the Abtalkh formations. The Abtalkh Formation, which is studied here, reaches thicknesses of up to 1750 m in the area. The formation is characterized by abundant, diverse, and poor to moderately well preserved calcareous nannofossil assemblages of Tethyan affinity. The assemblages were studied in detail in two sections in eastern Koppeh-Dagh, allowing construction of a precise biozonation for this stage. The Abtalkh Formation at sections in Abtalkh village and Padeha spans biozones CC20 to CC23a (UC15bTP to UC16). The results of this study indicate a late early to late Campanian age for the formation in the area. The most complete Campanian sequence is in the southeast, where the Padeha section is located. Nannofossil abundance and diversity decreases upwards, showing a trend from the base to top of the formation. Dominance of warm water taxa, and low abundance of high latitude taxa, confirm placement of the basin in low to mid palaeolatitudes during deposition of the formation.