The Walton family, which dominated the upper echelons of the Forbes list in recent years, tumbled in this year's ranking as stock in the world's largest retailer dropped more than 10 percent in the past year.
S. Robson Walton, known as Rob, who last year ranked 10th, fell to 19th with $15.8 billion. Christy Walton and Jim Walton tied for 17th with $15.9 billion each, while Alice Walton followed Rob Walton at $15.7 billion. Helen Walton, mother of the clan, did not make it into the top 20, landing at No. 21 with $15.6 billion.