BACKGROUND Rotaviruses are the most common agents of diarrheal illness of infants and young children. Gastroenteritis caused by rotaviruses is also more likely to be associated with severe dehydration compared to other viral gastroenteritis. OBJECTIVE We determined the G-serotype of rotaviruses circulating in the Metro Toronto-Peel County region, during the 1997-1998 winter. METHODS Stool samples were collected in Metro Toronto-Peel county region from November 1, 1997 to June 30, 1998. Rotavirus presence was demonstrated by electron microscopy and ELISA. A reverse transcription-polymerase chain reaction (RT-PCR) assay for the amplification of the near-complete RHA segment 9 of rotavirus was developed and used to determine the G-serotype by sequencing and phylogenetic analysis. RESULTS A total of 135 rotavirus isolates were used as templates for RT-PCR. Of these, four could not be amplified and one revealed a mixed infection. The G-serotypes could be unambiguously determined for the other isolates. Of these, 85 were of G-serotype 1, 41 of G2, 1 of G3, 1 of G4, and 2 of G9. CONCLUSIONS Our RT-PCR assay enables unambiguous determination of the G-serotype. The overall relative incidence of G-serotypes in Toronto is similar to that of other developed countries. Serotype G9 was present at low levels in Toronto.