Google Inc, the world's leading search engine, said on Monday it was concerned about the free flow of information on the Internet if Microsoft Corp were to succeed in acquiring Yahoo Inc.
Last month, Microsoft proposed buying Yahoo in a deal originally worth $44.6 billion, but Yahoo's board has rejected the offer, saying it was too low.
"We would be concerned by any kind of acquisition of Yahoo by Microsoft," Chief Executive Eric Schmidt told reporters.
"We would hope that anything they did would be consistent with the openness of the Internet, but I doubt it would be."
Schmidt pointed to Microsoft's past history and "the things that it has done that have been so difficult for everyone", but he did not elaborate.
Last year, a European court upheld a landmark 2004 decision that Microsoft abused the near-monopoly power of its Windows operating system to damage competitors, along with a 497 million euro ($695 million) fine.