Two major antigenic subgroups (A and B) have been described for human respiratory syncytial virus. The complete nucleotide sequence was determined for the fusion (F) mRNA of the subgroup B strain 18537 and the amino acid sequence of the F protein deduced, for comparison with the previously described sequences for the A2 strain of subgroup A. The F proteins (excluding the cleaved signal peptide) were 91% identical between the subgroups, consistent with the previously described high degree of antigenic relatedness. The greatest divergence occurred within the F2 subunit immediately preceding the cleavage activation site.