Around 13.6 billion searches were conducted in the US last month. Google sites accounted for 64.7 per cent, and Yahoo for 19.6 per cent, meaning that both had lost a 0.3 per cent hold on the market since June.
Microsoft, meanwhile, which launched Bing in June, gained 0.5 per cent to give it 8.9 per cent of US search queries.
The statistics follow an announcement in July that Microsoft and Yahoo will partner to take on Google.
Microsoft's Bing will now power Yahoo search, while Yahoo will use its solid relationship with advertisers to become the 'salesforce' for both companies' search advertisers, the firms said.
Microsoft and Yahoo claimed that the deal will accelerate innovation in search, and make Bing-powered search more personally relevant to users because of its increased advertising strength.
The V3.co.uk Labs Blog has a Bing versus Google comparison of the search engines' relative merits.