The New York Stock Exchange is investigating Oppenheimer Holdings, the Toronto-based holding company for securities brokerage and investment firms, on suspicion of market timing by several employees.

According to a filing with the Securities and Exchange Commission, Oppenheimer is preparing a formal response to the NYSE investigation explaining why the regulator should not take enforcement action against the firm for its alleged improper market timing of mutual funds by several former employees. Oppenheimer also said it has set aside funds to address the issue.