Google was used 63 percent of the time in August, compared to 61.9 percent in July, according to figures released Friday by comScore, which tracks online trends.
Yahoo came in second in August at 19.6 percent, down .9 percent from July. Microsoft garnered 8.3 percent of traffic in August, down .6 percent from a month prior.
ComScore's data was drawn from 11.7 billion total searches. It includes searches done on other Web sites employing certain search engines but does not include mapping, local directory and user-generated video sites, which are measured separately.
The slipping market share of Yahoo and Microsoft doesn't speak well for their efforts to invigorate their online services and brands.
Microsoft's efforts to acquire Yahoo earlier this year were due in part to its struggle in trying to catch up with Google and belief that Yahoo would bring it valuable engineering resources.
The two companies appear to have ceded ground to even more distant competitors in the search market. The Ask Network increased its share of searches to 4.8 percent in August from 4.3 percent a month before.
Also, AOL -- which has undertaken a revamp of its portal while also building an online advertising business -- moved up slightly, from a 4.2 percent share in July to 4.3 percent in August.
In separate statistics, comScore drills down into where searches are performed within a company's services, giving an indication of how popular certain offerings are.
For Microsoft, the news again isn't good. Searches on its MSN portal and Windows Live, an opening page that acts as a gateway to blogging and e-mail services and includes search, were down 7 percent between July and August.
Other notable statistics were a 3 percent rise in queries on Google's YouTube video service and a 9 percent increase in searches on MySpace, the social-networking site owned by Fox Interactive Media. Facebook saw an 8 percent rise in searches, according to comScore.