The deduced amino acid sequences of the outer capsid protein, VP7, of serotype G1 rotavirus clinical isolates collected over a 6 year period (1990)(1991)(1992)(1993)(1994)(1995) in Melbourne, Australia, were examined. Phylogenetic analysis characterized the sequences into two discrete clusters representing two of the four global lineages of human G1 VP7 proteins. Antigenic characterization using a panel of serotype G1-specific neutralizing monoclonal antibodies classified lineage II isolates (1990-1993) as monotype G1a while lineage I isolates were classified as monotype G1b (1993-1995). Examination of the sequences of the neutralization epitope regions of VP7 revealed a particular amino acid substitution at residue 94 in region A (Asp Ser/Thr) that correlated with lineage and monotype designation. Our results indicated that temporal genetic variation of the VP7 of serotype G1 rotaviruses was associated with changes in the antigenicity of these isolates.