Regulators worried about opacity and volatility in exchange-traded funds are for the first time requiring them to disclose the Wall Street brokers they rely on to smooth trading in the securities.

The Securities and Exchange Commission approved on October 13 a rule that includes a requirement for ETFs to report which banks are the most active middlemen in the $2.4 trillion market. The move reflects SEC officials' concerns that they don't know enough about how the withdrawal of a few major traders could disrupt ETF prices for everyday investors, according to people familiar with the matter.