The tax agency encouraged taxpayers to file electronically, either through tax preparers or through computers using tax-preparation software. It said those using e-file who choose direct deposit can receive refunds in as little as 10 days, while those who owe taxes can file now and pay later.

Last year nearly 80 million returns, almost 57 percent of the total, were filed electronically. The IRS electronic filing program began in 1986 with 25,000 returns.

The agency cautioned on Thursday that as many as 13.5 million taxpayers who use five forms related to the alternative minimum tax will have to wait until around Feb. 11 before filing.

In December, after the IRS had prepared its forms for the 2008 filing season, Congress enacted legislation to shield some 20 million taxpayers from getting hit by the AMT, which was originally aimed at only a small number of very rich people. The IRS says it needs the additional time to reprogram systems to comply with the new law.

The agency also said that taxpayers using the IRS Free File program, now in its sixth year, can begin filing on Friday.

The program, for taxpayers having an adjusted gross income of $54,000 or less, is run by the IRS and a consortium of tax-preparation software companies.

This year taxpayers can choose from 19 software companies, including two offering Spanish-language programs. The programs are accessible only through the IRS Web site, www.irs.gov.