More than four out of 10 web searches conducted in the US are now done using the Google search engine, figures show.
Google's share rose to 42% in February, up 6% on the same month in 2005, while rival Yahoo saw a 4% drop to 27%, according to tracking firm ComScore.
Usage of MSN's search engine fell by 3% to 13% said ComScore, while Ask had a 1% rise to 6%.
In a separate report, analysts Merrill Lynch said Google's advertising revenues was expected to rise.
The Merrill Lynch report said: "We expect Google's increased market share in search queries, and better monetisation of queries to lead to increased share of ad dollars relative to its competitors."
A report by RBC Capital Markets said Google could go on to capture a 70% market share.
It said: "The question really comes down to how long it could take."