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Microsoft saw its share of the market slip to 9 percent, down from 10.1 percent a month earlier. Google, meanwhile, increased its lead, accounting for 55.2 percent of Web searches, up from 53.7 percent in March. Yahoo grew its share to 21.9 percent, up narrowly from the 21.8 percent share it held in March.

The April results return to a familiar pattern of Google gaining ground at the expense of its chief rivals, although Microsoft had been doing a bit better in recent months. In its January earnings conference call, Microsoft executives indicated that they were not pleased with the company's search results.

AOL remained in fourth place for April, but saw its share slip to 5.4 percent from 5.8 percent in March, while Ask.com held steady in fifth place with 1.8 percent of search queries.

In an interview at Microsoft's Strategic Account Summit two weeks ago, a top executive expressed hope that Microsoft was starting to make inroads in search, but said that it might not post gains every month.

"I'm not even going to say it's a trend yet," Chief Advertising Strategist Yusuf Mehdi said, referring to the fact that Microsoft had posted share gains for each of the past three months. "I'm not going to predict that that's the bottom and now it's all up, but that's momentum."