Chief executives of public companies have long feared Carl C. Icahn. The 88-year-old investor made a name and billions for himself by questioning the decisions and strategies of corporate leaders and agitating for change at companies like Apple, RJR Nabisco and Netflix.

But now Mr. Icahn is under intense scrutiny from Wall Street investors, who are rapidly selling his company’s stock. In the past year and a half, shares of Icahn Enterprises, his publicly traded investment company, have dropped more than 75 percent, losing nearly $20 billion of value. After dropping more than 30 percent since mid-August alone, it now trades at about $11 a share, its lowest level in more than two decades.

Mr. Icahn owns roughly 86 percent of the shares, so he has personally lost billions of dollars, too.

“There’s a confidence game and he’s lost the confidence of investors,” said Don Bilson, who focuses on activist investing as the head of event-driven research at Gordon Haskett Research Advisors.

Some Wall Street investors are now worried that the stock’s continuing fall could threaten the health of the entire company and that it could be forced to sell companies it holds. Icahn Enterprises holds a mix of public stocks, real estate and other investments, according to interviews with Mr. Bilson and several other market watchers.

Investors have been questioning whether Mr. Icahn himself has been selling his stock. He has taken out personal loans using his stock as collateral. Banks that offer these loans typically have strict requirements related to the value of a company. A sharp drop in a stock price could force a lender to sell shares.