On the face of it, Senator Harry Reid’s explosive but flimsily sourced claim that Mitt Romney paid no income tax seems preposterous. Mr. Romney has denied it, and without his returns no one can say for sure. But for someone who makes millions of dollars a year, would it even be possible?

Evidently it is.

It so happens that this summer the Internal Revenue Service released data from the 400 individual income tax returns reporting the highest adjusted gross income. This elite ultrarich group earned on average $202 million in 2009, the latest year available. And buried in the data is the startling disclosure that six of the 400 paid no federal income tax.

The I.R.S. has never before disclosed that last fact.

Not even Mr. Romney, with reported 2010 income of $21.7 million, qualifies for membership in this select group of 400. But the data provides a window into the financial lives and tax rates of the superrich. Since the I.R.S. doesn’t release data for the tiny percentage of Americans at Mr. Romney’s income level, the 400 are the closest proxy.

And that data demonstrates that many of the ultrarich can and do reduce their tax liability to very low levels, even zero. Besides the six who paid no federal income tax, the I.R.S. reported that 27 paid from zero to 10 percent of their adjusted gross incomes and another 89 paid between 10 and 15 percent, which is close to the 13.9 percent rate that Mr. Romney disclosed that he paid in 2010. (At the other end of the spectrum, 82 paid 30 to 35 percent. None paid more than 35 percent.) So more than a quarter of the people earning an average of over $200 million in 2009 paid less than 15 percent of their adjusted gross income in taxes.