Despite the struggling U.S. economy, the nation's richest Americans retained their wealth.

In the 27th edition of The Forbes 400, the list of the richest Americans -- topped by Bill Gates -- starts at $1.3 billion for the second straight year.

Gates, who retained his spot atop the list of the nation's richest, has a net worth of $57 billion. He surpassed Warren Buffett, who at $50 billion took Gates' crown for a few months earlier this year.

No. 3 on the list was Lawrence Ellison, who is worth $27 billion. He is followed by four members of the Walton family, who are related to Wal-Mart pioneer Sam Walton. The Waltons include Jim Walton ($23. 4 billion), S. Robson Walton ($23.3 billion), Alice Walton ($23.2 billion) and Christy Walton ($23.2 billion).

New York Mayor Michael Bloomberg ($20 billion) was the biggest gainer on this year's Forbes list. His net worth rose $8.5 billion after he bought back a 20 percent stake in his firm Bloomberg L.P. from Merrill Lynch.

Newcomers to the list included 24-year-old Mark Zuckerberg, who founded Facebook; fertilizer tycoon Alexander Rovt, car dealer and art collector Norman Braman; and Patron tequila founder John Paul DeJoria.