Carl Icahn has acquired a nearly 6% stake in oil and gas producer Talisman Energy Inc. (TLM, TLM.T), a move the activist investor said may lead to conversations with management about board seats and strategic alternatives.

The news sent shares of the Calgary, Alberta, company up 6.1% to $13.53 in after-hours trading.

Talisman has been hit hard by a slump in North American natural-gas prices. The company, which depends on natural gas for the bulk of its earnings, has posted a string of disappointing financial results. The stock has languished after falling sharply in 2011.

In a tweet Mr. Icahn issued on Twitter, which was verified by an Icahn representative, the investor said he may have conversations with Talisman's management regarding strategic alternatives and board seats, among other topics.

The tweet came around the time of a Securities and Exchange Commission filing that showed Mr. Icahn had acquired roughly 61.6 million of Talisman's shares.

Talisman spokeswoman Phoebe Buckland, in a prepared statement, said the company appreciates "constructive input from shareholders" and takes "their views seriously."

"We are committed to acting in the best interests of the company and give due consideration to constructive recommendations for strategies or actions that have the potential to increase shareholder value," Ms. Buckland said.

-Ben Lefebvre contributed to this article.

Write to John Kell at john.kell@wsj.com

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