The Duopoly by the Numbers
Citadel Securities handles approximately 28% of all U.S. equity trades and more than 40% of retail equity order flow. Virtu Financial handles another 20-25% of retail order flow. Together, these two firms execute the majority of every retail investor's trades in the United States — a concentration that would prompt antitrust scrutiny in virtually any other industry.
Competition or Collusion?
With only two major competitors, the dynamics of PFOF pricing are opaque. Brokers select between Citadel and Virtu (and a handful of smaller market makers) based on per-share payment rates and execution quality metrics. The question of whether this bilateral negotiation between duopolists reflects competitive pricing or tacit coordination is one that neither the SEC nor DOJ has investigated publicly.
Entry Barriers
The wholesale market-making industry has extremely high barriers to entry: technology investment requirements, regulatory capital requirements, and the network effects of existing broker relationships all make it nearly impossible for new entrants to challenge Citadel and Virtu's duopoly. This structural lock-in amplifies the harms of PFOF because investors have no practical ability to route their orders outside the duopoly.