The maize stripe virus (MStV) major noncapsid protein (NCP) gene was characterized, and the location of the NCP gene was identified among the 5-RNA, 18-kb genome. A 12-amino-acid sequence of the NCP was compared with nucleotide sequence data for MStV RNAs 3 and 4 and was found to align perfectly within a 528-nucleotide open reading frame (ORF) of RNA 4. The amino acid composition of purified NCP was almost identical to that deduced from the putative coding region. The deduced NCP molecular weight was 19,815, very similar to that determined by SDS-PAGE analysis of the purified NCP. In vitro transcription and translation analysis of the cDNA representing this region showed unequivocally that this region encoded the NCP. Primer extension analysis using a synthetic oligonucleotide complementary to a sequence near the 5' end of the coding region revealed that the NCP ORF is located 61 nucleotides from the 5' end of RNA 4.