Partial sequences of the G protein gene of 33 isolates from antigenic group B of human respiratory syncytial virus were determined. Phylogenetic analysis indicated that the evolutionary pattern of group B viruses is similar to that previously described for isolates of antigenic group A, including worldwide distribution of related viruses and co-circulation of viruses from different lineages during the same epidemic. Dominance of AG+GA over UC+CU transitions was observed when G sequences of group B viruses were compared, as previously found in viruses from antigenic group A. Interestingly, differences in protein length, determined by the usage of alternative termination codons, were more pronounced in group B than in group A viruses. Changes in protein length correlated with the classification of viruses in different lineages. Thus, mutations that determined termination codon usage seem to have played an important role in the diversification of group B viruses.