The nucleotide sequence of the leader and the gene encoding the nucleocapsid protein (NP) of mumps virus Miyahara strain have been determined. The leader sequence is 55 nucleotides in length and the NP gene is 1845 nucleotides in length, exclusive of poly(A). The NP gene codes for a protein of 549 amino acids, with a calculated molecular weight of 61,365. For epitope mapping, a series of NPs from which C-termini were serially deleted were expressed in vitro from five mRNA constructs and were examined by radioimmunoprecipitation assay (RIPA) with eight nonoverlapping monoclonal antibodies (MoAbs) against the mumps virus NP. It was found that seven out of eight MoAbs reacted with the NP synthesized in vitro. Five recognized the epitopes located within the C-terminal 74 amino acids region and one within the adjacent 64 amino acids upstream. The epitope of the remaining one was in the N-terminal half of the NP.