Influenza virus nucleoprotein (NP) was purified from the virus and found to be virtually free from RNA. The morphology of the negatively stained NP was studied using electron microscopy. The monomer protein was found to be a small rod with dimensions of 62 by 35 A. However, most of the protein was found to exist in polymeric forms ranging from trimers to large structures that were morphologically indistinguishable from the intact viral RNP. Each monomer has two sites for NP-NP contacts at one extremity of the rod. The consequences of this finding for crystallization studies, for in vitro studies on NP-RNA interactions and the possible consequences for in vivo ribonucleoprotein particle assembly are discussed.