AdMob is a leader in mobile advertising and already serves ads for some applications running on Google's Android mobile operating system.

In a statement, Google said that AdMob has made "exceptional progress" so far in the mobile advertising market, which is thought to be in its early development stages.

The deal fills a hole for Google, which has mainly focused on mobile search ads, it said on Web pages that it put up to offer further details about the acquisition. AdMob has largely focused on display ads and in-application ads.

Google and its competitors including Yahoo and Microsoft have been competing to gain traction in the mobile market, which they see as having potential for significant revenue streams. Already there are more mobile-phone users in the world than computer users, so they believe that serving ads to mobile users can build a new source of revenue.

On the questions-and-answers page about the deal, Google said that it currently makes a very small amount from mobile ads relative to its overall revenue, "but the prospects for this space are excellent." Usage of its search engine by mobile users has grown five-fold over the past two years, Google said.

The acquisition indicates how serious Google is about mobile and about display advertising, said Greg Sterling of Sterling Market Intelligence.

It's also a large acquisition for Google. "This is one of those 'major' acquisitions that CEO Eric Schmidt said would occur only once a year," Sterling said.

While Google said that it doesn't see any regulatory concerns over the deal, it took pains on the Web pages to describe how much competition there is for mobile ads and why the deal is good for users. Mobile customers may end up with more free mobile applications because developers may be able to support their work through ads, Google argues.

It also points out that Google's competitors including Yahoo, AOL and Microsoft have also recently made acquisitions of mobile advertising companies.

Google also said that it will extend its privacy protections to its mobile advertising plans, perhaps in anticipation of privacy concerns around the deal. Earlier this year two privacy groups asked the U.S. Federal Trade Commission to regulate how mobile markers can use consumers' personal information. They cited AdMob among companies that they say collect information from mobile users without adequate notice.

Both Google and AdMob have approved the acquisition.