Today is the deadline for some late taxpayers to file their tax forms.

While most taxpayers filed their forms with the Internal Revenue Service by April 15, those who requested an extension by that date were automatically given until Aug. 15 to file.

An estimated 3.6 million individuals filed the required Form 4868 requesting an extension this year. Many of those taxpayers have filed their forms since then, so the IRS has no estimate of how many are due today.

Taxpayers who received extensions and cannot meet today's deadline should have filed Form 2688, stating their reasons for requesting an additional two months to file. These requests are not granted automatically. The IRS must determine that the taxpayer has "reasonable cause" for the additional extension to be granted.

Those individuals who request additional time by filing Form 2688 today "are proceeding at their own risk" because they have not given the IRS time to approve the request, said Ernest Acosta, an IRS spokesman.

Late-filing penalties are levied on those who miss the original deadline without applying for an extension, or those who miss later deadlines. Last year, 1.3 million taxpayers had to pay penalty fines totaling $355 million for filing late.

Taxpayers can meet today's deadline by hand delivering their forms to any local IRS office or to any post office in time to get an Aug. 15 postmark.

Owners of small businesses, consultants and other self-employed workers with Keogh retirement plans can fund their plans up until the time they file their forms, so long as the plan was opened during 1984, Acosta said.

Holders of Individual Retirement Accounts had until April 15 to fund their plans.

Many Keogh holders, however, continue to struggle with one particularly complex form. Form 5500-C is required this year for all holders of Keogh plans, which allow self-employed persons to defer taxes on income set aside for their retirement. Holders of the most elaborate plans have been required to file the form for some time, but from now on the requirement applies to all Keogh holders.

The form seeks information about the individual plan. But because plans vary so greatly, and because the form is so complex, the IRS has been unable to offer assistance in filling it out, said Ted Kern, director of IRS's Baltimore District, which includes the District of Columbia.

IRS officers cannot direct taxpayers in filling out Form 5500-C because most taxpayers lack detailed information about their Keogh plans, Kern said. Much of the form must be filled out by the "plan sponsor," the financial institution that sold the individual Keogh plan, Kern said. Keogh holders should fill out what they can and submit it for completion to their plan sponsors, which may be banks, insurance agents, brokerage houses or other institutions, he said.

Kern said there have been problems with some IRS officers simply turning away Keogh holders without answering questions or advising them to take the forms to their sponsors. The IRS office in Wheaton temporarily posted a sign telling Keogh holders that the IRS could not help them with Form 5500-C. "I told them to take that sign down," Kern said, adding that he had given the office instructions two weeks ago on how to handle questions about Form 5500-C.

The original July 31 deadline for Form 5500-C has been extended until Sept. 30.