A consensus sequence for the P protein gene of the RSN-2 strain of respiratory syncytial (RS) virus was obtained by PCR amplification of cDNA obtained by reverse transcription. This established that the extent of sequence variation between two P genes of strains of antigenic subtype B is similar to that among A strains, confirming the conservation of P genes within subtypes and the divergence of the two antigenic subtypes of RS virus. The P protein of RS virus exhibits anomalous electrophoretic mobility with respect to its molecular weight. In vitro transcription and translation of RSN-2 strain cDNA possessing single point mutations revealed that substitutions involving charged amino acids in the carboxy-terminal region had a marked effect on the electrophoretic mobility of the P protein.