We evaluate the enforcement of China Walls—information barriers within conglomerates. We focus on July 2018, when the SEC tightened enforcement around broker-dealer subsidiaries, and on the 25 million Israeli Shekel trades over 2018–2024. Our difference-in-differences design compares a dealer’s affiliate, connected, and unrelated funds on days when the dealer likely obtained valuable information. Before July 2018, dealers share information with their affiliate and connected funds. After July 2018: the sharing ceases only with the affiliate funds; price discovery slows; trading costs fall for affiliate funds and rise for other funds. Tightly enforced walls eliminated 99.0% of affiliate funds’ event-day profits.