Foraminiferal tests from two sediment cores from the western Arctic Ocean were investigated for evidence of an evolutionary transition from Cassidulina teretis Tappan to Cassidulina neoteretis Seidenkrantz during the Pleistocene. Previous results based on core material from the North Atlantic and the Nordic Seas suggested minute differences between these test types and resulted in the emendation of C. neoteretis, as well as its designation as a Pleistocene biostratigraphic marker. This study identifies both species throughout most of the western Arctic foraminiferal record, without a clear pattern of evolution from one to another, except for the uppermost part of the stratigraphy (late Pleistocene). These results indicate that the C. teretis/C. neoteretis transition was more time-transgressive than previously believed and that these foraminifers may be ecophenotypes rather than different species. The stratigraphic use of C. neoteretis is likely to be site-specific and caution should be taken in its application across the entire Polar region.