Apple Inc
Apple shares closed at the session high of $665.15, conferring on the Silicon Valley giant a market capitalization of $623.5 billion, surpassing Microsoft's 1999 value of $620.8 billion, according to data provided by S&P Dow Jones Indices.
Microsoft would still hold the record if the numbers were adjusted for inflation.
Apple's stock is up almost 9 percent so far in August, as Wall Street bets on the September 12 rollout of the latest version of the iPhone, the device that revolutionized the mobile industry.
Apple climbed even as fellow technology heavyweight Facebook Inc
Apple's stock usually rallies in the runup to major product launches, among the most heavily watched events on the annual tech calendar. The iPhone is the company's biggest product, yielding half or more of its sales.
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Sources have said the company will take the wraps off a larger version of its iPhone on September 12. Some analysts also think it intends to announce a smaller version of its iPad, to safeguard its market share, as rivals from Google Inc
But Bernstein Research's Toni Sacconaghi warned that questions remain about the availability of components for both the iPhone and the iPad, which in the past has constrained Apple's product shipments.
"A key question for the launch will be Apple's expected rollout schedule," the analyst wrote on Monday. "Apple's intention is to continue to ramp offerings as quickly as possible, but the company's ability to do so remains a key near-term question."
POLAR OPPOSITES
Apple became the biggest public company in the world when it overtook Exxon Mobil
"Everyone loves a winner; if you play the quick trade be careful," said Howard Silverblatt, senior index analyst at S&P Dow Jones Indices in emailed comments. "If you are an investor, check the fundamentals and business plans, and avoid the hype in your decision."
Facebook's stock has gone the opposite direction in the past month as investors worried about the company's ability to make revenue grow. Last week, some early investors were given the go-ahead to sell for the first time since Facebook's May 18 IPO. Several similar "lockups" will expire through to the end of the year.
Facebook rebounded above $20 in afternoon trade after Capstone's upgrade, based on a combination of a more attractive valuation since its decline, and good long-term advertising prospects.
"It seems to be down around levels that people who didn't like the deal thought it was really worth. And now it seems to have stabilized," sad Eric Kuby, chief investment officer, North Star Investment Management Corp in Chicago.
It may have "found a level which seems more of a better price for people valuing the company in terms of the future."