Technology titan Bill Gates has been listed by Forbes magazine as the wealthiest American for the 19th year in a row, with a fortune of $66 billion, up $7 billion from last year.
There was no change in the order of the top five richest from a year earlier.
The total wealth of the US super-rich grew 13% to $1.7 trillion, with the top 400 worth an average $400 million more in 2012.
The group’s assets are worth as much as one eighth of the US economy, and grew much faster than the economy at large.
According to the Forbes 400 list of the richest people in America, the average net worth of a person on the list was $4.2 billion.
In second place with a fortune of $46 billion was investment guru Warren Buffett, who is chairman and chief executive of the insurance conglomerate Berkshire Hathaway.
He was followed by Larry Ellison, head of software maker Oracle Corp, worth $41 billion.
David and Charles Koch of the energy and chemical business group Koch Industries were tied in fourth place with $31 billion.
The majority of those on Forbes‘ list became richer in 2012. Two hundred and forty-one members of the group saw their wealth increase, while just 66 saw it shrink.
Casino magnate Sheldon Adelson and financier George Soros dropped from the ranks of the top 10 into 12th place compared with a year ago.
But the biggest drop was seen by Facebook founder and chief executive, Mark Zuckerberg, who fell from 14th to 36th place in the wake of a disappointing stock market listing of his company.
He lost nearly half his fortune, which is now worth an estimated $9.4 billion.
Four members of one family – the heirs to the Walmart fortune – are in the top 10.
Top 10 Forbes list:
1. Bill Gates, co-founder and chairman of Microsoft, $66 billion
2. Warren Buffett, chairman and chief executive of Berkshire Hathaway, $46 billion
3. Larry Ellison, co-founder and chief executive of Oracle, $41 billion
4. Charles Koch, chairman and chief executive of Koch Industries, $31 billion
5. David Koch, co-owner and executive vice-president of Koch Industries, $31 billion
6. Christy Walton & family, heiress to Walmart fortune, $27.9 billion
7. Jim Walton, heir to Walmart fortune and chairman of Arvest Bank, $26.8 billion
8. Alice Walton, heiress to Walmart fortune, $26.3 billion
9. S Robson Walton, heir to Walmart fortune, $26.1 billion
10. Michael Bloomberg, founder and principal owner of Bloomberg LP, $25 billion.