New York-based Pershing Square typically buys large stakes in a handful of big companies and agitates executives and directors to make changes to boost shareholder returns.
ADP said Friday that Ackman's Pershing Square Capital Management informed the company that it wants Ackman nominated to the 10-member board, along with four others, and it wants to push back the August 10 deadline for nominating directors.
Mr. Ackman is no stranger to ADP, having invested in it from 2009 to 2011.
The payroll company said it declined to extend the deadline.
Shares in Chipotle, on which Pershing Square spent about $1.17 billion past year, are down more than 17 percent since the stake was disclosed.
He also has a billion-dollar bet against multilevel marketing firm Herbalife, alleging that the nutrition supplement seller is a pyramid scheme.
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"Since Carlos Rodriguez became CEO almost six years ago, ADP's total shareholder return of 202% is well in excess of the S&P 500 TSR of 128% - and is many multiples of Pershing's TSR of 29%".
"ADP is open to constructive input from our shareholders, and our Board respects the right of shareholders to nominate directors", Automatic Data Processing said in a statement.
He has forced changes, cheered by investors, at companies like Canadian Pacific Railway, as well as Air Products and Chemicals Inc.
Pershing owns 8 percent of ADP, which processes checks for about 20 percent of the US work force. Pershing Square now has around $10-billion in assets under management.
"ADP appears to be strongly pushing back against Pershing Square's Bill Ackman, unlikely to accede to any corporate governance or financial/operational demands", Wells Fargo & Co. analyst Richard Eskelsen, who has a market perform rating on the shares, wrote in a note to clients.
Shares in ADP rose almost 2.8 percent in premarket trading on Friday, to $114.85.