The complete nucleotide sequence of RNA segment 5 of the influenza C/California/78 (C/Cal/78) virus was determined by using cloned cDNA derived from viral RNA. The gene contains 1809 nucleotides and can code for a protein of 565 amino acids with a molecular weight of 63 525. The RNA 5 protein of the influenza C/Cal/78 virus possesses two short regions which share a high degree (60-83%) of sequence homology with the nucleoproteins of influenza A and B viruses. These and other structural features of the RNA 5 protein suggest that RNA 5 of influenza C viruses codes for the nucleoprotein. The data also suggest that influenza C viruses are orthomyxoviruses, but that they are more distantly related to either type A or type B viruses than are influenza A and B viruses to each other.