Thirty samples were examined to document the nannofossil assemblage and determine the biostratigraphic zonation. Samples were taken from a borehole drilled at Sisseton (NE South Dakota). 92 species of nannofossil were determined and photographed. Biostratigraphic study of calcareous nannofossils allowed recognition of Late Early Campanian to Early Late Campanian age for the studied section