Nasdaq OMX (NDAQ) is considering the launch of a third U.S. equities exchange in 2010 as its junior stock platform gains traction, according to people familiar with the discussions.

A new exchange would give Nasdaq OMX another venue to experiment with pricing and test-drive new products as it works to reclaim lost market share in U.S. stock trading.

Nasdaq OMX has been holding talks with retail trading firms, proprietary trading shops and other participants as it shapes the effort. It would use an exchange license acquired with last year's purchase of the Philadelphia Stock Exchange, the sources said.

Most cash equities trading activity at Nasdaq OMX occurs on the Nasdaq Stock Market, the 38-year-old exchange that carries a higher price tag for trade executions.

But recent growth at Nasdaq OMX BX, a smaller and more cheaply priced market launched in January and aimed at high-frequency traders, has the company considering a third platform that would fall somewhere in between.

Nasdaq OMX weighs the move as its U.S. equity trading business has been seriously dented in the past year.

Smaller electronic rivals BATS Exchange and Direct Edge have taken over nearly a quarter of the market by offering fast executions and low prices, while off-exchange stock trading continues to grow.

With more venues competing for traders' business, Nasdaq OMX's overall market share has fallen from 30% a year ago to an estimated 22% this month, after stabilizing in June.

Rival NYSE Euronext (NYX) has been hit as well, with its market share falling to 28% from 34% last September.

Some of the stabilization at Nasdaq OMX came via a money-losing price plan on the Nasdaq OMX BX platform, formerly known as the Boston Stock Exchange, acquired last year.

The inverted pricing scheme helped lift BX's market share from below 1% in May to 2.6% last month, according to figures from Nasdaq OMX.

Business on BX has continued to rise this month, even after Nasdaq OMX raised prices on Sept. 1. Nasdaq OMX BX now accounts for around 3.2% of the market, with a record 381 million shares changing hands Sept. 15.

A third Nasdaq exchange would likely carry a pricepoint between the Nasdaq Stock Market and Nasdaq OMX BX, according to people familiar with Nasdaq OMX's thinking, though fees would probably lie closer to the BX end.

A new cash equities market from Nasdaq OMX also would further crowd the U.S. landscape.

Kansas City, Mo.-based BATS Global Markets last week announced a second platform of its own called BYX; rivals NYSE Euronext (NYX) and Direct Edge, like Nasdaq OMX, already operate multiple venues.

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