Several hundred D.C. taxpayers were incorrectly told late last month that they were delinquent on their income tax payments because an antiquated city computer system failed to properly credit payments made last year.

"I hate to send out even a single bill that is incorrect," said Natwar M. Gandhi, deputy chief financial officer for tax and revenue. "But it has taken us 20 years to mess up our [computer] systems, and it will take another two or three years to replace it."

Tax officials are not sure exactly how many taxpayers were sent the inaccurate notices, but they said the mistake involved people who are self-employed or retired, or for some other reason make estimated income tax payments each quarter.

Some estimated payments made during two quarters in 1998 were improperly credited by the computer system to 1997, leading the computer to list such taxpayers delinquent for 1998. Bills went out on May 29 asking these taxpayers to resolve the debt, resulting in about 550 complaints, D.C. officials said.

Bill Herron, a downtown resident who received one of the inaccurate notices, said he does not understand why the city did not disclose the problem before now.

"I called in last week, and I could not get them to answer the phone," he said. "It just compounds the problem."

About 60,000 D.C. residents make estimated tax payments each quarter. But city officials believe the number of delinquent notices sent out by mistake is much smaller, because they noticed the computer problem in early April and corrected it before it resulted in inaccurate bills for most of those filers.