(Updates with additional details and data)

Nasdaq OMX Group Inc. (NDAQ) said Thursday its matched-volume market share of NYSE-listed equities slipped again in December from the previous month.

Nasdaq's matched-volume market share - trades executed at the same level as the market's opening reference price - of NYSE-listed stocks was 20%, compared with 22.3% in November and 18.3% a year earlier. Overall volume was 21.6 billion shares.

Average daily matched volume of Nasdaq-listed securities was 762 million shares, down 21% from November.

Trading has increasingly moved away from the Nasdaq and NYSE to alternative venues as electronic trading continues to proliferate.

Nasdaq's matched share of all U.S. equities was 27.2% in December, down 2.3% from the previous month as NYSE Euronext (NYX) reclaimed trading lost earlier in 2008, and alternative trading venues like Bats Trading and DirectEdge posted their own volume gains.

NYSE Euronext reported last week that its U.S. average equity volume rose 30% during December, while its European stock business was "stable" and the Liffe derivatives arm was off 19%.

Bats, which has converted to an exchange, reported last week that its own U.S. market share grew to 10.46% in December. Direct Edge claimed 12.26% of U.S. equities trading in December, pulling ahead of Bats in terms of volume.

With competition among equity exchanges intensifying on both sides of the Atlantic, the surge in U.S. stock trading has helped alleviate weakness in Nasdaq OMX's European stock and derivatives operations.

Trading in equity and fixed-income derivatives products rose 5% on a month-to-month basis in December, with a major gain in long-term interest-rate products.

However, both product lines are down from the year-ago period, tracking broad declines in derivatives trading seen at CME Group Inc. (CME) and Deutsche Boerse's (DB1.XE) Eurex unit.

The exchange group's share of U.S. options trading inched upward to 19.8% of the total market, but overall options trading activity fell 17% from November.

Nasdaq's shares were recently $20.71, down about 0.2% for the day.

-By Kerry E. Grace, Dow Jones Newswires; 201-938-5089; kerry.grace@dowjones.com; and Jacob Bunge, Dow Jones Newswires; 312-750-4117; jacob.bunge@dowjones.com

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