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Abstract. Middle Jurassic calcareous nannofossils and ammonite assemblages from the Baghamshah Formation are described here, cropping out east of Semnan locality (Central Iran Basin, Iran). Low diversity and abundance characterize the nannofossil assemblages, which are dominated by genera belonging to the Family Watznaueriaceae. The calcareous nannofossil assemblages lack the usual markers for the Callovian interval, making difficult the age attribution. Ammonite assemblages display low diversity and poor preservation. From a biostratigraphical point of view, nannofossil assemblages would suggest that the material studied belongs to the NJT12 Zone from the Tethyan / sub-Mediterranean province and to the NJ12b subzone -lower NJ13 Zone from the Boreal province, correlating to the ammonite Tethyan / sub-Mediterranean Gracilis Zone and to the Boreal / sub-Boreal Koenigi -Calloviense Zones, yielding a Lower Callovian biostratigraphical distribution.