The lawsuit, filed in federal district court in Colorado by JP Enterprises--which runs the online dating service lovecity.com--accuses Yahoo and the other companies of bidding to appear on Google's results pages when users query on "lovecity" and the related terms "lovecity.com" and "www.lovecity."

Yahoo runs its own online personals service, as do at least two of the other companies named in the lawsuit--HDVE, which operates True.com, and Spark Networks, which runs a host of Internet dating services including AmericanSingles.com and JDate.com. Little information was immediately available about the role of the third company sued, Insight Direct USA.

Spokespeople from Yahoo and Spark Networks declined to comment, while Insight Direct and True.com didn't return telephone calls. News of the lawsuit was first reported Thursday in the Denver Post.

JP Enterprises maintains that Yahoo and the other companies are violating the JP Enterprises trademark by using it to trigger ads for their own, competing dating sites.

In its complaint, JP Enterprises says that the ads cause consumer confusion, hurt the "lovecity" brand, and drive down sales. JP Enterprises "spent hundreds of thousands of dollars and expended years of effort in advertising, promoting and developing the trademark lovecity.com," the lawsuit states. The complaint goes on to accuse Yahoo and the other companies of "tarnishing the goodwill and reputation" of lovecity, and "causing lost sales."

Yahoo and Google both have been sued for trademark infringement for allowing companies to bid to appear when users type rivals' names into the query box.

Yahoo settled with a case in late 2004 with insurance giant Geico, but Google took the matter to trial and prevailed on the relevant issues.

In an ironic twist to this case, Yahoo's own search division, Yahoo Search, in February stopped allowing companies to use rivals' trademarks to trigger search ads.

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