The 2000 Census is just getting underway and already there is a numbers problem. The Census Bureau is mailing 120 million misaddressed letters informing every American household that the important questionnaire will soon follow.

The mistake tacked on an extra digit to the beginning of each street address. Instead of "1600 Pennsylvania Avenue," for example, the address will read something like "31600 Pennsylvania Avenue."

Census Bureau Director Kenneth Prewitt blamed a contractor for the "regrettable mistake" but said it should have been caught by the government's quality control procedures. The U.S. Postal Service, after assessing the situation, assured the bureau yesterday that the letters will get to their intended addresses.

While the mistake created a public relations problem for the statistical agency, Prewitt emphasized that it would not hinder the $6.8 billion decennial head count. The census forms that most Americans receive next month are correctly addressed, he said.

"I do consider this a mistake that is extremely unfortunate from the point of view of public response," Prewitt said yesterday. The error "did not affect operations but does have a public image issue attached to it."

The address glitch appeared on letters the Census Bureau is sending for the first time, part of a campaign to reverse declining public response to the count. The letters also offer people the option of receiving a form in Spanish, Chinese, Korean, Vietnamese or Tagalog.

Prewitt said officials checked yesterday with cognitive psychologists, who regularly advise the bureau on direct-mail techniques, and were told that people will not notice the error. Apparently, most people are so accustomed to seeing their addresses that they would not notice a minor disparity.

Census Bureau officials did not name the contractor. The company was hired by the Government Printing Office and received $5.6 million for the job--the only contract it had for the census. The contractor changed procedures after a successful test run that was monitored, officials said, and the flawed envelopes were not seen before they went to the post office.

One person familiar with the situation said that the General Accounting Office is looking into the cause of the error and that the Commerce Department's inspector general also may be asked to investigate.

One government official said a postal manager in New England discovered the address error Tuesday night. The first letters--20 million intended for rural households--are supposed to be mailed March 1, but some went out earlier. The second batch of letters, 100 million for the rest of the country, are to be mailed March 6.

Postal officials told the Census Bureau that the forms, addressed to "Resident," would be delivered to the correct address because the Zip codes and bar code identifications on the envelopes were correct. The correct codes allow the post office's automated machines to sort and ship the letters to the right delivery points.

But U.S. Postal Service officials issued a special alert this week to its letter carriers, warning them so they would not be confused when they found incorrectly addressed letters in their mailbags. Prewitt praised the postal agency for "exemplary" actions beyond what it would have done for a private mailing.

The faulty mailing prompted the Census Bureau to check its other printing jobs, including the census forms, which already had been checked more thoroughly than other mailings.

"We spent the entire day rechecking every one of those major print runs, and they tested fine," Prewitt said.