Activist investor Daniel Loeb's Third Point Capital LLC has taken a stake of less than 5 percent in Hertz Global Holdings Inc (HTZ.N), the second largest U.S. car rental company, CNBC reported.
Hertz shares rose as much as 9 percent on Tuesday morning.
The company said on Monday that it had adopted a 'poison pill' plan after observing "unusual and substantial activity" in its shares.
The shareholder rights plan will be activated if any person or group acquired 10 percent or more of Hertz's stock.
Third Point was not immediately available for comment.
The fund, which owns 7.48 percent total stake in Sony Corp (6758.T), has been pushing for the electronics giant to partially spin off its lucrative entertainment business.
Third Point also invested $1 billion in Softbank Corp (9984.T) in November.
Hertz shares were up 8 percent at $28.07 on the New York Stock Exchange.
The stock had risen 58 percent this year to Monday's close, while shares of rival Avis Budget Group Inc's (CAR.O) almost doubled in the period.
(Reporting by Rohit T.K. in Bangalore and Christian Plumb in New York; Editing by Kirti Pandey)