Investors' growing disdain for actively managed mutual funds has produced a disturbing consequence: Ownership of American public companies is increasingly concentrated in the hands of just four giant index-fund managers. Through the end of 2003, the world's four largest index-fund firms -- Barclays ( BCS), State Street ( STT), Fidelity Management and Vanguard Group -- in aggregate held an average of 12.6% of the 20 largest U.S. companies on behalf of their clients. Together, for instance, this elite band owns 15% of IBM ( IBM), 15% of Citigroup ( C) and 14.5% of Johnson & Johnson ( JNJ). Barclays, a British bank, has emerged as the first among near-equals. It has paired its new iShares exchange-traded funds business with its long-established pension fund indexing business to become one of the largest single holders of U.S. companies, if not the largest -- about $1 trillion worth. Fourteen of the 20 largest U.S. public companies now count Barclays as their largest institutional owner.