Two domains involved in RNA synthesis have recently been found within the N-terminal 77 amino acids of the Sendai virus P protein. One domain is required for RNA synthesis per se and has properties in common with the transactivation domains of cellular transcription factors. The second domain is thought to be specifically required for the nascent chain assembly step in genome replication. We have further mapped this second domain by the construction of chimeric and deleted P proteins to amino acids 33 to 41 of P and by examining the abilities of these P proteins to support DI genome replication in vivo. Using glycerol gradient sedimentation, we have shown that this domain is required to form a stable complex with unassembled NP (P-NP0) and to prevent NP from assembling illegitimately, i.e., independently of the concurrent assembly of a nascent viral genome. Since the P-NP0 complex represents the functional form of unassembled NP which is delivered to the nascent chain during genome replication, and since amino acids 33 to 41 are not required for the stable interaction of P with the assembled NP of the nucleocapsid, this chaperone function of P is not required for mRNA synthesis or the RNA synthesis step of genome replication.