The sequences of the P mRNA and protein of strain 18537 of antigenic subgroup B of human respiratory syncytial virus were determined by sequencing cloned cDNAs of intracellular mRNA. Comparison with the corresponding sequences of the A2 strain of subgroup A showed that there was extensive sequence identity at both the nucleotide (80% identity) and amino acid (90% identity) levels. The P proteins contained a single divergent region (52% amino acid identity) flanked by highly conserved domains (96% identity). The previously observed differences in electrophoretic mobilities between the P proteins of subgroup A strains and certain subgroup B strains could not be attributed to differences in the polypeptide Mr of the primary translation product.