The S&P/TSX Composite Index was down 42 points in choppy trade mid-morning Monday, with industrials down 1%, the biggest decliner.
The energy sector is the second biggest decliner, shedding 0.7%, as oil prices tracked lower following US threats to impose harsher sanctions against Iran.
The best performer was Information Technology, up 0.4%.
In economic news, the Canada-US trade talks collapsed at the 11th hour after Canada walked away from the negotiating table over the weekend. Both sides blamed each other for the breakdown in negotiations.