A large proportion of intracellular Sendai virus (SeV) M proteins is phosphorylated, but in mature virions the M protein is not phosphorylated or dephosphorylated. Phosphorylated M protein in cells is bound to the cytoskeletal components more firmly than unphosphorylated M protein. Thus it has been hypothesized that M protein phosphorylation plays an important role in the virus life cycle, especially in the step of maturation. Here, a transient expression-mutation experiment of the M gene demonstrated that a change of the Ser residue at the 70th position from the N-terminus to Ala (S70A) totally abolished M protein phosphorylation, strongly suggesting that this residue is phosphorylated. The mutated M gene was then placed in the corresponding region in the cDNA plasmid which generates a full-length antigenome SeV RNA, and a mutant SeV M-S70A was successfully recovered from the cDNA. This mutant virus was indeed defective in M protein phosphorylation but did not differ at all from the wild-type SeV recovered from the parental cDNA either in the replication kinetics and plaque morphology in cultured cells or in in vivo replication and pathogenicity for mice. We thus concluded that no phosphorylation of the M protein was required for SeV replication either in vitro or in vivo.