WASHINGTON (AP) _ It's a con artist's dream. After years of learning to lock doors and install burglar alarms, Americans are being told it's their patriotic duty to open their homes to strangers.

It's the U.S. Census 1990, and some swindlers are taking advantage:

-An 80-year-old North Carolina woman was tied up and robbed by two men she thought were census workers.

-People claiming to be census employees charged Houston residents $50 after helping them complete their forms.

Census officials say such scams have to be expected.

''When you do something this massive, there's someone who will figure an angle to get money from it,'' said Noe Balli of the census' regional office in Dallas. He warned, ''Anybody who wants money, they're obviously not census employees.''

There are 200,000 to 230,000 official census enumerators combing the countryside, knocking on doors of those who failed to complete the form sent out this spring, according to Mark Mangold of census headquarters.

Each worker is armed with a red, white and blue badge identifying the holder by name and I.D. number as a ''U.S. Census Enumerator.'' In addition, each worker carries a large black satchel with a red, white and blue patch that says U.S. Census 1990, said Ray Bancroft of the census promotion office.

To keep from becoming the victim of an imposter, people should demand to see identification and, if they have questions, they should call the local census office for verification or call the police before allowing the person inside their home, census officials said. Bancroft noted that impersonating a census worker carries a maximum sentence of three years in prison and a $1,000 fine.

There have been isolated incidents of con artists during every census, Bancroft said, and this year is no different. However, FBI spokeswoman Kelley Cibulas said there is no reason for panic because the impostors have been few in number.

But some people have not been wary enough.

In the North Carolina case, the woman initially allowed only one man into her home and was having a ''nice conversation'' with him when the doorbell rang and the man told her it was his partner, said Tom Smith Jr. of the Charlotte, N.C., office.

''They tied her up and commenced taking some valuable things from the house as well as her car,'' Smith said. ''They tied her hands and feet and she sat on the couch and watched them take the things.'' She was not injured, he said.

In Houston, people identifying themselves as census workers went to homes and helped the occupants complete the questionnaires. Then they charged them $50 for the help, Balli said. The so-called census workers didn't even have copies of the questionnaire but used those sent to the residents, he said.

Another Houston resident complained, meanwhile, that someone identifying himself as a census worker telephoned seeking credit information and credit card numbers, Balli said.

In Colorado, a woman identifying herself as a census worker asked an elderly woman resident for money. The request was refused and the elderly woman called officials, said Ron Ritschard of the Denver census office. There were no other reports of similar incidents, he said.

Nevada residents complained about a mailing that said ''Official Census Form'' on the envelope but actually was sent by a clothing manufacturer, said Bob Clingman of the regional census office in Seattle.

''There are some innocuous questions, and they say if you send it in on time, you can get a special price on two pairs of our new jeans for only $29.95,'' Clingman said. The case was not pursued because the manufacturer's name was included on the mailing envelope, he said.

Not everything that appears to be a scam is a scam, noted B.J. Welborn of the regional census office in Boston.

For example, people shouldn't necessarily be suspicious if a person identifying himself as a census worker seeks someone's Social Security number, she said. While the census under way this year does not require the number, it is needed for some surveys being conducted by the Census Bureau.

However, Welborn warned people to be alert if the person asks for bank account numbers, as no census survey seeks such information.

Sometimes it's the census workers who are victimized, officials said.

About two months ago, a fearful homeowner shot an enumerator, who survived, said Balli of Texas.

The assailant ''was an elderly gentleman in Mississippi who had been vandalized so many times, and he saw this fellow on his property,'' Balli said.

The tires of one census worker's car were slashed in northern New Jersey, said Stephanie Lowe of the bureau's Philadelphia office.