E*Trade Financial Corp's stock slid by more than 7 percent and Schwab was down by more than 3 percent, their worst performance since Schwab last lowered its fees on Feb 2. All three stocks charted deeper losses earlier in the day.
Shares in TD Ameritrade were hit the hardest, as it derives about 42 percent of revenue from trading fees, the biggest exposure of the three listed companies.
Boston-based Fidelity's online brokerage business has 17.9 million accounts and $1.7 trillion in total client assets. The company said its efficient processing of trades and other services make it the best value.
Fidelity also cut rates for investors who trade on margin, or with borrowed money from the brokerage.
San Francisco-based Schwab has 10.2 million active brokerage accounts and $2.83 trillion in client assets.
"Please don't miss the bigger picture here," Schwab's chief financial officer, Joe Martinetto, said in a statement calling the fee cuts a "growth strategy."
"This is a company that is performing extraordinarily well."
The fee-cut announcements offered a reminder of how much trading costs have come down for investors.
Robinhood, a commission-free trading app for retail investors, said they were "happy" that Fidelity lowered its fees.
"Ideally, they would have eliminated them altogether, along with the required $2,500 account minimum," it said in a statement.