Activist investor Carl Icahn has increased his stake in Chesapeake Energy Corp., which gave the energy company's shares a slight boost in Monday morning trading.

A regulatory filing with the U.S. Securities and Exchange Commission on Friday showed that Icahn held 66.5 million shares in Chesapeake as of Aug. 1, which represents a 9.98 percent stake. That is up from an 8.98 percent stake, based on a November filing.

Chesapeake shares climbed as high as $25.58 Monday as the stocks of some of its peers fell. But by early afternoon Chesapeake's stock price dipped 3 cents to $24.98.

Icahn has always been known as an influential investor, but his actions lately seem to have added pull.

Last week, Icahn said via Twitter that he has acquired a large but unspecified stake in Apple Inc. and that he thinks the company should do more to revive its stock price. He said that he had spoken to the company about boosting its share repurchase plans and the stock immediately jumped, closing at its highest level in seven months.

He has also been involved in a very public fight to oppose Michael Dell's efforts to take Dell Inc., the company he founded, private. Investment fund Southeastern Asset Management is also fighting against the effort. The opposing arguments have been compelling enough to prompt Dell's board to delay the shareholder vote on the deal three times.

Chesapeake, based in Oklahoma City, is going through its own changes. The company said last week that four of its top executives have left the company as part of an ongoing reorganization effort. That is in addition to the company's co-founder and former CEO Aubrey McClendon leaving the company in April and senior vice presidents Tom Price and Henry Hood leaving in May.