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.

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