Icahn, one of the world's most outspoken activist investors, spent $470.5 million for the 38.8 million shares he owns, according to a regulatory filing.
He said he may seek a board seat, something he has done at many other companies. Last year Hertz adopted a so-called poison pill to prevent any one shareholder from gaining control of the company as activist investors began circling.
Hertz said in a statement on Wednesday that it welcomed dialogue with its shareholders, was implementing new procedures and controls, and had made "important additions" to its accounting and finance teams over the last few months.
On Wednesday the company's stock slumped 13 percent when Hertz withdrew its full-year financial forecast, blaming a shortage of cars due to motor industry recalls and an accounting error.
Hertz announced in March that it would spin off its equipment rental unit and earlier in the year speculation mounted that Icahn had built up a stake in the company.
But it was not among the roughly two dozen U.S. stocks that Icahn said he owned at the end of the second quarter when he released his quarterly filings last week.
At 78 when most people are living off their life's investments, Icahn is still casting around to make new bets and is busier than ever in shining the spotlight on management teams he feels are underperforming.
On Monday he told Reuters that Family Dollar Stores, a company he owns stock in, has wasted hundreds of millions of dollars by trying to avoid being bought by rival Dollar General, as Icahn has suggested.
And Icahn is not the only investor expressing concern about management. A spokesman for Fir Tree Partners, which owns a 3 percent stake, said "the CEO has had some serious missteps."
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