It has always been clear under Canada's Competition Act that any merger can be challenged by the Competition Bureau after its closing. In practice, however, that rarely ever occurs. So it surprised many observers when, on January 24th, 2011, the Bureau filed a notice of application with the Competition Tribunal seeking an order to dissolve a completed and non-notifiable merger between two hazardous waste landfill owners on the basis that it substantially prevented competition in the relevant market. This case, along with a recently announced intention to amend the Merger Enforcement Guidelines, may signal a more aggressive and expansive approach to merger reviews by the Bureau. Originally published by the CCH Competition Law Clearinghouse.