Replication of negative-strand RNA viruses relies on two components: a helical ribonucleocapsid and an RNA-dependent RNA polymerase composed of a catalytic subunit, the L protein, and a cofactor, the P protein. We show that the measles virus (MeV) C protein is an additional component of the replication complex. We provide evidence that the C protein is recruited to the ribonucleocapsid by the P protein and map the interacting segments of both C and P proteins. We conclude that the primary function of MeV C is to improve polymerase processivity and accuracy, rather than uniquely to antagonize the type I interferon response. Since most viruses of the Paramyxoviridae family express C proteins, their primary function may be conserved.