Income inequality has company—make that companies. A new wealth gap is opening among U.S. corporations, where cash holdings are growing more concentrated as the rich get richer.

Eighteen American businesses held 36 percent of corporate wealth in 2013, up from 27 percent in 2009, according to a report from Standard & Poor's. The bottom 80 percent have lost ground, with just 11 percent.

The top 1 percent is a Who's Who of multinationals, including Microsoft Corp., Google Inc., Coca-Cola Co., Apple Inc., and Ford Motor Co., that reap a big share of profits from non-U.S. sales. Because tax law discourages moving that money back to the U.S., cash is piling up abroad and companies are taking novel steps to adapt, including borrowing against those assets to finance operations at home.