ABSTRACT: Stratigraphic studies, based on lithology, diatoms, and mollusks, have suggested different chronostratigraphic placements for formations in the Pisco Basin. This study focusses on a stratigraphically important, calcareous nannofossil-bearing unit between the Chilcatay and Pisco formations. The Eocene-Pliocene Pisco Basin is located at low latitude (-14 degrees S) in a forearc basin in central coastal Peru. The objective of this study is to use nannofossil biostratigraphy to constrain the age of an interval for ongoing stratigraphic work in the region.We analyzed nannofossils from two sections located on the northern end of the Cerros Yesera de Amara trend and in Cerro Las Tres Piramides. A tuff bed above the top of the section is dated to 17.70 +/-0.24Ma based on a single Ar-Ar age from biotite. Analysis of productive samples from the two locations show rare to common calcareous nannofossil occurrence with poor to moderate preservation. Nannofossils show strong dissolution and/or overgrowth in almost all samples. Based on the first occurrence (FO) of Helicosphaera carteri in association with Cyclicargolithus abisectus, Reticulofenestra bisecta, and Triquetrorhabdulus carinatus, we suggest that the assemblage recovered represents an age of earliest Miocene, Zone NN1 (CN1b). Studies from the equatorial Pacific region indicate rare and discontinuous occurrence of Reticulofenestra bisecta (<10 micrometers) in late Oligocene and earliest Miocene (NN1), making it difficult to determine if its occurrence in the assemblage is caused either by low productivity or by reworking. The absence of Discoaster druggii might suggest that the assemblage is not younger than NN1, although Discoaster druggii tends to be rare and difficult to find even in well preserved sediments.