Can shareholders sue a company's auditor for negligence in Ontario?

Sometimes, but Ontario courts have generally limited an auditor's duty of care to the corporation itself rather than to every individual shareholder or investor who later relies on the audited financial statements to make a personal investment decision. The reasoning is that an audit is commissioned for the company's own governance and reporting purposes, not as a guarantee to every person who might one day read the statements before buying shares or extending credit, so extending a duty that broadly would expose auditors to open-ended liability.

That doesn't mean shareholders never have a claim - a derivative action brought on the corporation's behalf, or a claim where the auditor's report was prepared with the specific, known purpose of being relied on by a particular shareholder or class of investors for a specific transaction, can support liability. Whether a duty exists in your situation depends heavily on how the audit was commissioned, who it was addressed to, and what the auditor knew about how it would be used. Given how fact-specific and narrow this area is, getting an early assessment of whether a duty was actually owed to you personally, versus only to the company, is essential before investing in a claim.