This paper deals with the optimal enforcement of the competition law between the merger and anti-cartel policies. We examine the interaction of these two branches of the competition policy given the budget constraint of the competition agency and taking into account the ensuing incentives for firms’ behavior in terms of choice between cartels and mergers. We are thus able to conclude on the optimal competition policy mix. We show for instance that to the extent that a tougher anti-cartel action triggers more mergers taking place, the public agency will optimally invest only in control fighting for a tight budget, and then in both instruments as soon as the budget is no longer tight. However, if the merger’s coordinated effect is taken into account, then when resources are scarce the agency may optimally have to spend first on controlling mergers before incurring the cost of fighting cartels.