SAN FRANCISCO — Internet powerhouses Yahoo! and eBay are joining forces in an alliance that further defines the battle lines in an online brawl with rivals Google, Microsoft and AOL.

Under the multiyear partnership announced Thursday, Yahoo! and eBay will draw upon each other's strengths in online advertising, payments and communications so they can connect with even more Web surfers than they already do.

Shares of both companies jumped on the news, reflecting investor expectations that the deal will open up even more moneymaking opportunities for two of the Internet's oldest and most successful businesses.

Yahoo! operates the world's most trafficked Web site with 402 million unique users and ranks second in the lucrative search advertising market behind Google. eBay is the e-commerce leader, with nearly 200 million users of online auctions and another 73 million U.S. account holders of its payment service, PayPal.

Nearly 113 million people in the United States visited Yahoo! or eBay or both destinations in April, according to Nielsen/NetRatings.

Under their partnership, Yahoo! will become the exclusive provider of display advertising throughout eBay's Web site and will add some search-generated ads, as well.

In another new twist, Yahoo!'s brand and search engine will be blended into an eBay Web browser toolbar that has been downloaded by 4 million users so far.

eBay's PayPal service will become the preferred payment provider for purchases made on Yahoo's site, which provides a wide array of shopping, auctions and subscription services.

And eBay's Skype Internet telephone service will be used to explore building another marketing vehicle that would allow advertisers to connect with prospective customers on the phone instead of through their Web sites.

Thursday's announcement followed several weeks of Wall Street speculation about the mating dance among the Internet's heavyweights. At one point, there was even talk that Microsoft wanted to buy Yahoo.

Coupled with Google's recent $1 billion investment in AOL, the Yahoo!-eBay partnership also figures to intensify the pressure on Microsoft to find an ally as it tries to become a more formidable player in Internet advertising.

Industry analysts Thursday identified Amazon.com, the Internet's top retailer, and MySpace.com, an increasingly popular teen hangout, as the most logical candidates for a Microsoft alliance.

Unlike the Google-AOL deal, Yahoo! and eBay aren't swapping any money. Instead, they are pooling their resources in an effort to boost their revenue beginning next year — a prospect that provided a lift to their recently slumping stocks.

eBay shares surged $3.68, or 12 percent, to close at $33.88. Yahoo!'s shares rose $1.13, or 3.6 percent, to finish at $32.92.

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