For the first time, Microsoft's Bing has surpassed Yahoo to become the number two search engine in the U.S., according to Tuesday data from Nielsen.

Last month, Bing nabbed 13.9 percent of searches, up 2 percent from the previous month and up 30 percent from the same time period last year. Yahoo had 13.1 percent of searches in August, down 8 percent from July and down 18 percent from last year.

Google still dominated the search category with 65.1 percent of searches, up 1 percent from the previous month and last year. Ask.com came in at number four with 2.1 percent and AOL Search rounded out the top five with 2 percent.

As part of a 2009 search deal between Yahoo and Microsoft, Bing last month officially started powering Yahoo's search results in the U.S. and Canada.

"If we combined Bing-powered search in August pro-forma, it would represent a 26 percent share of search," Nielsen said in a blog post.

Nielsen said its stats include search queries from users; they do not include contextual searches, hovers, or in-text searches.

Statistics about search engine market share differ depending on the source, however.

Last week, Hitwise said that Yahoo had 14.28 percent of searches in the U.S. in August, compared to 9.87 percent for Bing, and 71.59 percent for Google.

ComScore data for July 2010, meanwhile, put Bing at number three with 11 percent of the market, while Yahoo was at number two with 17.1 percent. That represented a 0.4 percent increase for Yahoo month-over-month, but no change for Bing, comScore said.