NYSE Euronext said activity on its U.S. stock exchanges fell 19.8% in September from August.

The drop, to 2.6 billion shares traded every day on average, followed a month of “heightend market volatility.” On Aug. 4, Standard & Poor's downgraded U.S. government debt for the first time, leading to four days of huge swings in market prices and more activity than normal throughout the month.

The volume of trading was up 12.3%, however, from September 2010, the operator of the New York Stock Exchange said.

NYSE Euronext’s share of trading in NYSE-listed stocks was 36.3%, up from 36.2% in August but down from 38.4% a year ago.

Globally, its derivatives volume was mixed as well. Daily volume of 9.5 million contracts in September 2011 was an increase of 23.2% versus the prior year, driven by a 32.1% increase in U.S. options. But it was down 8.4% from August.

In Europe, its daily volume of 4.8 million derivatives contracts was up 13.5% compared to September 2010 and up 7.0% from August.

NYSE Euronext is in the final stages of attempting to merge with Deutsche Boerse, operators of the Frankfurt exchange and the Clearstream post-trade services organization.

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