Marc Andreessen won't be charting America Online's technology future after all.

In a surprise move, America Online Inc. of Dulles announced yesterday that the Silicon Valley hyper-achiever is stepping down as chief technology officer after only six months on the job. He'll shift to a part-time role as an adviser to the company, focusing on investments in new technology companies.

Andreessen, who was AOL's first chief technology officer, will be replaced by William J. Raduchel, 53, a former chief strategy officer for Sun Microsystems Inc., a California company with which AOL has a strategic alliance.

Andreessen was only 22 when he co-founded Netscape Communications Corp. in 1994. It went on to become one of the most innovative Internet companies that Silicon Valley has produced. AOL acquired it in March for $10 billion and depicted Andreessen's move to the Dulles-based company as a sign of technology leadership and a smooth melding of the two firms.

He bought a large house in McLean and commuted between Virginia and Silicon Valley. The Washington area technology community embraced the tall, rapid-talking, onetime computer science student as a new local star.

AOL officials denied yesterday that his departure as chief technology officer was a setback. But industry analysts said the move could be a sign that the AOL-Netscape transition is not going as smoothly as hoped. "These are two companies with very different cultures," said Bill Whyman, an Internet analyst with securities firm Legg Mason Wood Walker in Washington. "It shows that the integration of AOL and Netscape will be difficult, because Andreessen is a critical signal to all the people who have stayed at Netscape."

Many Netscape technologists left the company rather than work for AOL, which at Netscape had a reputation as a consumer entertainment operation, not an advanced technology shop.

Andreessen yesterday brushed off suggestions that he had differences with AOL. "If this was a cultural issue, I'd be full up quitting," he said.

But he said it had recently become clear to him that he could not manage his interest in nurturing start-up companies with the vast responsibility of vetting all technology at AOL.

"It's hard to have a full-time job anywhere and also be involved in start-ups," Andreessen said. "I don't intend to work full-time for another company again."

Andreessen has personal investments in Silicon Valley firms Replay Networks Inc. and Accompany Inc.

Over the past two months, Andreessen said he and AOL chief executive Steve Case have had a running dialogue about what's important at the company and what Andreessen's new role should be. "Steve believes large parts of this industry are going to consolidate . . . but I also think new opportunities will open up," Andreessen said.

Others said that while Andreessen is great at big-picture thinking, he never was a good day-to-day manager. "AOL's bureaucracy needs a hands-on manager, not so much a visionary," Bethesda Internet analyst Gary Arlen said.

Andreessen doesn't contest that point. "I'm not the guy you're going to have in there negotiating terms or price," Andreessen said.

Several analysts said the move could in fact have been long-planned, with the goal of replacing Andreessen after he had helped smooth some of the transition. "It looks a little pat," Arlen said. "Marc could have been a placeholder while they were waiting for the Sun guy to come in."

Last month Andreessen filed with the Securities and Exchange Commission to sell 940,680 shares of AOL, worth about $88 million, although he still has a large stake in the company.

For his part, Raduchel said his discussions with Case have been taking place over a short time.

Andreessen said although he'll be spending most of his time in Silicon Valley with visits to other tech spots such as Seattle and Austin, he'll still come to Washington often to network and look at new companies.

He has been speaking regularly in the area and has joined the Capitol Investors, a local group of the tech elite who meet for monthly dinners and investment pitches. In October, Andreessen plans to host a fund-raiser for Vice President Gore's presidential campaign.

CAPTION: In his new job, Marc Andreessen will focus on start-up investments.