This paper develops a theory of the centralization of firms engaged in multi-market collusive agreements. A centralized organization (called the unitary or U-form) allows price coordination across several markets, whereas with decentralized (the multidivisional or M-form) firms the probability that the antitrust authority will find evidence of collusion on one market while investigating the other is lower. We show that the firm’s choice of internal structure depends to a large extent on product substitutability and the instruments used by the antitrust authority.