A quantitative and qualitative study of within-sample variation in the genus Sphenolithus Deflandre indicates the existence of significant polymorphism in this group of nannofossils. Qualitative comparison between two samples of different ages from different localities reveals two distinct ultrastructural morphologies indistinguishable in the light microscope. Statistical treatment of the quantitative data shows that the same variation exists in both samples. A number of heretofore distinct taxa are interpreted to be variants of only two species