Google's domestic market share rose to 42.3 percent in February, up from 36.3 percent a year earlier, ComScore said.
Yahoo's search market share in the United States fell to 27.6 percent from 31.1 percent a year ago, while Microsoft's MSN fell to 13.5 percent from 16.3 percent and Time Warner's America Online fell to 8 percent from 8.9 percent.
IAC Search & Media's Ask.com, whichlast month, rose to 6 percent from 5.3 percent.
Analysts predicted continued gains for Google and Ask.
"We see little to stop Google from reaching 70 percent market share eventually; the question, really, comes down to, 'How long could it take?" RBC Capital Markets analyst Jordan Rohan wrote in a research note Tuesday.
Merrill Lynch analyst Lauren Fine predicted that Google's increased market share and better monetization of queries will lead to an increased share of ad dollars relative to competitors in the first half of this year.
"Furthermore, we believe the company's clear strategic focus on search will continue to provide a competitive advantage for the company over the next several months as leading competitors struggle to overhaul/improve their search platforms," she wrote in a research note.
Fine said she is optimistic that rebranding and marketing investments at Ask will result in further share gains this month.
Meanwhile, Yahoo's improvements to relevancy and search monetization "appear to have had minimal effect on market share trends," she said. In addition, it will take a few more months to determine if, introduced just over a year ago, is "improving the consumer experience," she wrote.