The polls close in the riding east of Toronto at 9:30 p.m. ET.
Watch this page for live updates as the results roll in. CBC News Toronto at 11 will also have full coverage of the byelection results.
The riding has been without an MP since Flaherty's death in April.
Flaherty was the MP in Whitby-Oshawa for eight years. For much of that time, he had also served as the federal finance minister, though he resigned from cabinet a few weeks before his death.