At the very top, those making more than $1 million a year, the data showed that from 2004 to 2006 the number of audits rose 77 percent, from almost 9,600 to 17,000. But more than half of those audits were only letters asking for documentation.

The middle class was lightly audited in recent years because it relies mostly on wage income, which is reported by employers and from which taxes are withheld. The I.R.S. has told Congress that it captures 99 percent of wage income, but only about 70 percent of income in which there is little or no independent verification of the figures that people report on their tax returns.

Middle-class Americans most likely to have their tax returns examined under the new strategy are those who own a business, even a side business, or are landlords or have investment income. There is little or no independent reporting of such income; the I.R.S. has proposed increased verification and some withholding of payments to independent contractors to reduce cheating, but Congress has not moved on any of those suggestions.

Middle-class taxpayers who file a Schedule C — freelancers, consultants and very small businesses — are three times as likely to be audited as those in the same income group with no such business income.

The I.R.S. is also increasing scrutiny of people whose returns show they have bought into any of the growing number of schemes sold by people who teach, falsely, that wages are not subject to tax. Some customers of these schemes have received prison terms of more than 10 years, and the Justice Department is pursuing civil and criminal cases against scores of tax fraud promoters.

Blended families are also more likely to be audited. The I.R.S. said that it is finding that families created after a divorce, or the death of a spouse, often have children who are claimed on more than one tax return.

The I.R.S. reduced audits in only one category — farms, especially those with income of more than $100,000. Audits of these larger farms fell by 28 percent from 2,150 audits in 2000 to 1,547 audits last year. The I.RS. said it cut back because a growing number found no additional taxes were due.