The nucleotide sequences of the cDNA copies of the mRNA coding for the nucleocapsid proteins (NPs) of human parainfluenza viruses type 4A (PIV-4A) and type 4B (PIV-4B) were determined. The copy of PIV-4A NP mRNA contained 1885 nucleotides encoding a protein with a calculated molecular weight of 62,561. The same number of amino acids with a similar molecular weight (62,425) were predicted for the PIV-4B NP protein. Comparisons on the nucleotide sequence and the amino acid sequence of NP protein between these two subtypes revealed extensive homologies in the nucleotide sequence (87%) and in the amino acid sequence (93%). Furthermore, a conserved region with about 100 amino acids was observed between PIV-4s and other paramyxoviruses, Newcastle disease virus (NDV), Sendai virus, mumps virus (MuV), PIV-3, BPIV-3, measles virus (MV), and canine distemper virus (CDV), indicating a common ancestor for these nine viruses. Our data also indicated that the PIV-4 NP proteins were more closely related to MuV and NDV than to other parainfluenza viruses, PIV-3, BPIV-3, and Sendai virus. Interestingly, the NP protein homology between PIV-4s and the morbillivirus group, MV and CDV, was slightly higher than that between PIV-4s and the parainfluenza viruses, PIV-3, BPIV-3, and Sendai virus.