(FORTUNE Magazine) – On Sept. 11, when two planes crashed into the World Trade Center and the U.S. financial markets slammed shut, the world's economies depended for a moment on Roger Ferguson. As vice chairman of the Federal Reserve, Ferguson, 51, operates as Fed Chairman Alan Greenspan's right-hand man. And because Greenspan was in Switzerland when the attack occurred, Ferguson instantly became the primary U.S. financial-policy maker. First, he made a basic but crucial decision: The Fed would remain open. Then he took several steps to ensure that the Fed could provide liquidity if necessary. It worked: Bonds soon resumed trading, the dollar stayed afloat, and the global economy did not come to the grinding halt predicted by some pessimists. "That was an instance where the power of the public sector was clearly in view. No private-sector entity can provide liquidity on the scale that the central bank can," says Ferguson, with typical understatement.

Now, Ferguson didn't make our list of the most powerful black executives in America. Nor did Federal Com-munications Commission Chairman Michael Powell or Enron-hunting Deputy Attorney General Larry Thompson. We limited our list to those in the private sector in an attempt to glimpse the momentous changes under way in that traditionally white world. But that's not to say that Ferguson's sway over U.S. business is insignificant.

His interest in the Fed started early. Ferguson was just a nerdy ninth-grader in Washington, D.C., when, in 1966, he watched Andrew Brimmer become the central bank's first black governor. Ferguson went on to whip through Harvard, picking up an undergraduate degree, a law degree, and a Ph.D. in economics. He spent three years as a Wall Street lawyer in the 1980s before McKinsey snapped him up; he stayed for 13 years. Clinton appointed him to the Federal Reserve in 1997.

Ferguson's appointment created quite a stir in Washington. He was only the third black Fed governor in history, and when named vice chairman in 1999, the first black person ever to hold that office. There was talk he would succeed Greenspan at the top, though such an appoint-ment is unlikely to materialize now in Bush's Washington. As for his next step, he hedges, "I'd like to think that with experience in both the public and the private sectors, I can work easily in either." If he chooses the latter, perhaps we'll see him on our next list.

--Martha Sutro