The nucleotide sequence of the mRNA coding for the nucleocapsid protein (NP) of the paramyxovirus, human parainfluenza virus type 3 (PIV-3), has been determined. The NP mRNA was found to contain 1642 bases, excluding poly(A), and encode a protein of 515 amino acids, with a molecular weight of 57,823. Amino acid residues 1 through 420 of PIV-3 NP protein showed extensive sequence homology with the corresponding amino acids of Sendai virus nucleocapsid protein. There was virtually no homology between the last 95 amino acids. Comparison of the NP proteins of PIV-3, Sendai virus, measles virus, and canine distemper virus revealed, from amino acid residues 160 through 390, some conserved areas between the corresponding proteins of these paramyxoviruses. The 5' terminal sequence of PIV-3 NP mRNA (5'-AGGATTAAAG-3') was similar to the conserved sequence (formula; see text) found at the 5' termini of Sendai virus mRNAs. Both PIV-3 NP and Sendai virus mRNAs had a common 3' terminal tetranucleotide (5'-TAAG-3') preceding the poly (A) tail.