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Micron Technology can officially call itself a member of the David Einhorn club.

Shares in the memory chip maker jumped more than 6 percent on Thursday after Mr. Einhorn disclosed that his hedge fund, Greenlight Capital, owned a stake in the company. They closed at $19.98.

Micron was just one of several stocks to move in part because of the hedge fund manager’s widely followed commentary. Shares in Apple Inc. reversed losses earlier in the day after Mr. Einhorn offered encouraging words about the iPhone maker, while those in the St. Joe Company briefly tumbled after he reiterated his pessimism toward the real estate concern.

Other hedge fund managers moved stocks on Thursday as well. Hologic gained nearly 2 percent after Carl C. Icahn disclosed a 12.6 percent stake in the medical device maker, prompting the company to adopt a “poison pill” defense plan.

But it was Mr. Einhorn whose comments drew the most attention. In discussing Micron — a company he bet against more than a decade ago — the investor described the company as one of the survivors of the market for memory chips. It’s a tough industry with only three major players, but the upside remains big, as flash memory continues to be in high demand in devices like smartphones and video game consoles.

Mr. Einhorn also touched on Apple, which he pressed to return more money to shareholders earlier this year. The company agreed last spring to increase its stock buyback program fivefold, to $60 billion, and raised its dividend payouts, winning some praise from Mr. Einhorn (which happened to be a little tepid at points on Thursday: He described the company’s capital management policies as rising from a D- to a C+).

Now, Mr. Icahn is pressing for an even bigger $150 billion buyback. But Mr. Einhorn declined to comment on his fellow investor’s campaign.

“Carl’s view is not particularly important to mine, because I really don’t know what he’s thinking and I need to form my own view,” he told CNBC.