UPDATE 1-NYSE Euronext quarterly revenue falls a fifth

* NYSE net quarterly revenue off 21 pct to $559 mln

* Group Q3 net income down 42 pct to $108 mln

* NYSE Euronext earnings per share at $0.44

LONDON, Nov 6 (Reuters) - NYSE Euronext, the world's largest stock exchange operator, reported net revenue down 21 percent to $559 million for the third quarter, citing weaker trading in the United States and Europe.

The group said on Tuesday net income for the three months to the end of September was down 42 percent to $108 million, while earnings per share were off 38 percent at $0.44.

The results reflect weaker trading activity in the group's main equities and futures markets in New York, London and Paris because of ongoing concerns about the global economy and a strong third quarter last year.