We study the optimal timing of merger control by comparing the pre-and post closing enforcement. Mergers have both pro- and anticompetitive effects, and the parties’ (the agency and the merging …firms) veri…able information on them is endogenous: it depends on the timing of the merger control, as well as on some investment in evidence production. The ex post enforcement turns out optimal whenever the costs of providing veri…able information on both efficiency gains and market power are sufficiently low, regardless of whether the fi…rms know ex ante or not their true merger type.