US STOCKS-Wall St ends flat after six-day rally, energy rises

* Wisconsin Energy (NYSE: WEC - news) to buy Integrys for $9.1 bln

* Industrial shares among the weakest of the day

* ParkerVision (NasdaqCM: PRKR - news) plummets on its heaviest-ever volume

* Indexes: Dow down 0.2 pct, S&P down 0.1 pct, Nasdaq flat (Updates to close)

By Ryan Vlastelica

NEW YORK, June 23 (Reuters) - U.S. stocks ended a quiet session on Monday essentially unchanged as investors found few reasons to keep buying following a six-day rally in the S&P 500, though merger activity lifted energy shares.

The S&P was coming off its longest streak of advances since mid-April, and both it and the Dow had closed at records on Friday. While Wall Street's uptrend is still viewed as intact, investors are looking for catalysts to deliver more robust gains, and recent economic data has been mixed.

U.S. May existing home sales rose at more than twice the rate of growth expected, while a preliminary read on June manufacturing hit its highest since May 2010. Other surveys of manufacturers gave positive signals for the global economic outlook, but dark clouds remained over Europe.

"Right now we're digesting last week's gains and keeping an eye on all the economic data that's coming out this week," said Leo Grohowski, who oversees more than $184 billion in client assets as chief investment officer at BNY Mellon Wealth Management in New York.

"We're not likely to see a major pullback given healthy activity like buybacks and mergers, but today is a day of quiet consolidation."

Energy shares were the day's strongest group, up 0.4 percent. Wisconsin Energy agreed to buy Integrys Energy (NYSE: IEH - news) Group Inc in a deal valued at $9.1 billion. Integrys was the S&P 500's biggest percentage gainer, up 12.1 percent to $68.35, while Wisconsin fell 3.5 percent to $45.27.

The Dow Jones industrial average fell 9.82 points or 0.06 percent, to 16,937.26, the S&P 500 lost 0.26 points or 0.01 percent, to 1,962.61 and the Nasdaq Composite added 0.64 points or 0.01 percent, to 4,368.68.

Oracle Corp agreed to buy Micros Systems (NasdaqGS: MCRS - news) in a $5.3 billion deal. Micros rose 3.4 percent to $67.98 while Oracle (TLO: ORA-U.TI - news) rose 0.7 percent to $41.10.

Micron Technology Inc (NasdaqGS: MU - news) was little changed in heavy after-hours trading after reporting third-quarter results.

Following a 1.2 percent gain last week, industrial shares lagged on Monday, off 0.6 percent. General Electric Co fell 1.1 percent to $26.86 while Boeing Co (NYSE: BA - news) was off 0.9 percent at $130.85. Both are Dow components.

ParkerVision Inc plummeted 63 percent to $1.85 after a federal judge overturned a jury verdict that ordered Qualcomm Inc to pay $173 million for infringing patents. About 39 million ParkerVision shares traded hands, its heaviest day of trading ever.

About 48 percent of stocks traded on the New York Stock Exchange closed lower on the day, while about 54 percent of Nasdaq-listed shares ended in negative territory. About 5.07 billion shares traded on all U.S. platforms, according to BATS exchange data, below the month-to-date average of 5.63 billion. (Editing by Nick Zieminski)