UPDATE 1-High-frequency trading valuable, Nasdaq CEO says

* Greifeld says may be some "bad actors"

* CEO expects some adjustments to dark pools

By Jonathan Spicer

NEW YORK, Oct 2 (Reuters) - High-frequency trading, the split-second stock trading strategy that has come under increased political and regulatory scrutiny this past summer, is a "valued part of the market," the chief executive of Nasdaq OMX (NDAQ.O) said on Friday.

Robert Greifeld said that while there might be some "bad actors that need to be caught" among the growing ranks of these traders, overall they provided "a tremendous amount of liquidity" when markets tanked amid last fall's credit crisis.

"They are a valued part of the market," Greifeld, whose firm runs the all-electronic Nasdaq Stock Market, told a conference at Fordham University in New York.

The CEO added he expects the U.S. Securities and Exchange Commission -- which is probing high-frequency trading and other elements of the electronic marketplace -- to make some adjustments to dark pools, the 40-odd alternative venues where orders are anonymously matched so that investors can hide their intentions.

"I think you'll see some movement on dark pools," Greifeld said.

The SEC, a key player in the Obama administration's broad financial reform plan, is under pressure from some politicians and market participants to crack down on what some say are markets that give the advantage to sophisticated players at the expense of long-term investors.

The SEC has said it is investigating high-frequency trading, in which firms use computer algorithms to profit from arbitrage and market-making. More than 60 percent of total U.S. stock trading is estimated to involve high-frequency trading.

But the regulator has put more emphasis on probing dark pools, which it says could erode fair and transparent pricing in equities markets, as well as other parts of the marketplace. (Reporting by Jonathan Spicer; Editing by Gerald E. McCormick)