Electronic market-making firm Getco LLC continued to trade throughout Thursday's rampant market volatility, at a time when other high-frequency trading shops pulled out of the market.

"Just as we did in the 2008 financial crisis, Getco continued to provide liquidity and make two-sided markets both as a designated market maker on the NYSE floor and as a registered market maker on the electronic markets," said a spokeswoman for the Chicago-based firm.

Technology-focused liquidity providers such as Getco have taken on a key role in U.S. markets in recent years, accounting for an estimated two-thirds of trading activity in U.S. stock markets and around 40% of futures trade.

Reliant upon complex trading algorithms, the firms stand ready to take the other side of trades in stock, options, futures and foreign-exchange transactions. Advocates tout the firms' services as lowering trading costs for investors, while critics allege that the sheer speed of the firms' systems allows them to take advantage of slower-moving participants.

While Getco, among the largest high-frequency trading shops, remained in the market Thursday, other big shops pulled out.

Kansas City, Mo.-based Tradebot Systems Inc. told The Wall Street Journal Thursday that it shut down its trading operations when the Dow Jones Industrial Average dropped by 500 points, as the firm sought not to contribute to the volatility.

Critics have alleged that key participants stepping back from trading during Thursday's market plunge exacerbated the problem, by making it harder for investors to get into or out of positions.

Dennis Dick, a proprietary trader with Bright Trading LLC, said that as algorithmic trading shops seek to avoid the effects of a massively erroneous trade--said to be the root cause of Thursday's chaos--there are "less liquidity providers to cushion the blow."

"If these predatory market-making practices are allowed to continue, eventually there will be no real liquidity in the depths of the market and, when there is a market impact event, we're in big trouble," he said.

-By Jacob Bunge, Dow Jones Newswires; 312-750-4117; jacob.bunge@dowjones.com