Is that your garbage vigilante man?

If the folks back in Lexington could see Don Sundue now, they'd wonder why he left their cozy suburb for life in the big city.

This is what they'd see: a successful software consultant who picks through other people's garbage in his spare time.

Sundue doesn't enjoy it, but he's not ashamed, either. He's a Back Bay block captain, and for him, it's part of the job.

''This is sort of the grittier side of reality, and I'm sort of a hands-on guy," he said.

His title, which he shares with two dozen other volunteers, carries not a trace of authority. But that doesn't stop him from looking for misplaced garbage -- and, when necessary, tracing it to the people who put it there.

''We're sort of this vigilante group, helping to improve the situation," he says.

Most block captains don't go as far as Sundue. But they will correct their neighbors if they see them disposing of garbage improperly and will blow the whistle on repeated violators.

The program, part of the Neighborhood Association of the Back Bay, takes civic involvement to a new level. Typically, neighborhood groups focus their energies on demanding services from officials, blocking unwanted projects, and organizing crime watches. But in the Back Bay, neighbors police their own neighbors.

''Our language is always very, very positive," said Dianne O'Connell, a management consultant who organized the effort two years ago. ''It's never confrontational. It's always collaborative."

The program does more than monitor slovenly residents. It has created an efficient system for spotting other problems -- rat holes, graffiti, broken lampposts, cracked sidewalks -- that many urban residents accept as a fact of life.

''Having eyes and ears out there is the only way you can make a city work," said City Councilor Michael Ross, who represents the Back Bay and whose office is usually on the receiving end of the block captains' reports. ''It will never work if you just leave it up to City Hall to solve all these problems. We don't know where the pothole is, and where the missing brick is. But the residents do."

Neighborhood associations in Beacon Hill, the North End, and the South End are considering their own version of block captains. But it seems fitting that block captains originated in the Back Bay, where meticulous, affluent homeowners coexist with students and transient young professionals, for whom neighborhood cleanliness is often not such a high priority.

Not everyone appreciates the pointers. When one captain informed a motorist that he was parking his car illegally, she was told to get lost, O'Connell says.

''That's going to happen," she says.

And not everyone is up to the task. One captain has already quit after getting a hostile reaction from a neighbor.

Others, however, are willing to push the limits of neighborly conversation.

During one of his neighborhood patrols, Bob Beech, 71, noticed a large empty box and a discarded baby stroller on the sidewalk -- a definite no-no in the Back Bay, where garbage is left in the back alleys. Beech pointed out the violation to a man tending his garden next door, only to be met with indifference.

''I was having a little confrontation with him," recalls Beech, the co-owner of a travel agency. ''I said, 'You live here, you're an owner. Don't you like the front of your house to look nice? Do you want to live in a dump? . . . It's just a matter of being a good neighbor, that's all. Just give it some thought.' And I just walked away."

The block captain program evolved from a 2002 survey by the Neighborhood Association of the Back Bay that gauged residents' opinions of city services. Among those who responded, 70 percent thought the city's rodent control efforts were inadequate and 55 percent were dissatisfied with the city's enforcement of trash and recycling rules.

But it became clear to O'Connell, in the course of conducting the survey, that blaming the city would only go so far. The Back Bay's residents needed to shape up.

Many residents don't put their garbage out on the morning of pickup, but the night before, meaning it sits out for 12 hours or more. They don't secure it properly in bags. And they pile it on the corners of alleys, not behind their own buildings, often resulting in a mess when vehicles run over it. All three habits contribute to the Back Bay's other plague -- rats, which feast on accessible garbage.

O'Connell takes the high road, asserting that most violators don't know better. ''We have to educate, and then report."

Sometimes, violators are caught in the act. When a young woman tried to put out garbage on a recent Sunday night, she was met by block captain Anne Swanson, an editor at Houghton Mifflin and a 26-year Back Bay resident, who explained why it's a chore best left for morning.

''And she said she would do it next time," Swanson said. ''But she didn't do it that time, so it remains to be seen whether or not she's a convert. But she was a lovely young woman, and I'm sure she will think about it, at any rate."

But sometimes it takes detective work, which explains why Sundue sometimes finds himself going through people's garbage, looking for a letter with an address on it. If he finds incriminating evidence, he'll knock on the appropriate door.

Most block captains will leave this untidy task to the city's Inspectional Services Department. But Sundue says the city's enforcement usually ends with a ticket stuck to someone's door, and if the building is owned by an absentee landlord, no one claims it.

''We have found that it's much more effective to try to find the specific person who's ignorant of the trash laws and educate them, inform them, coerce them, whatever it is, and you get much better results that way," he said. ''One at a time you can get converts, whereas the fine system doesn't seem to do much."

So far, Sundue said, his overtures have never turned ugly. Residents typically blame someone else, particularly the building's management company. When that happens, Sundue contacts the company and explains the rules to them.

Not every block has a captain, though some blocks have more than one and some captains have more than one block. Altogether, 17 blocks -- most north of Newbury Street -- are spoken for. Newbury and Boylston streets, which are more commercial, remain largely unpatrolled.

Will block captains spread beyond the Back Bay? At the very least, block captains could help neighborhood groups with one of their big challenges: reaching out to residents who aren't members.

''I think one of the keys is one-to-one communication with neighbors, whether that's in the form of fliers or just conversations," said Tom Boyden of the Ellis South End Neighborhood Association.

Over in Beacon Hill, neighborhood leaders have talked about starting a similar program. But they have reservations.

''What you don't want to do is end up being a scold," said Judith Hughes of the Beacon Hill Civic Association. ''It's a fine balance. . . . Maybe it's not so bad to have your neighbors say, 'Hey, I saw you put your trash out a little early the other night.' Whether people would feel comfortable with that, that's something I'm sure you'd have to figure out."

But Mary Beth Karr of the North End/Waterfront Residents Association is intrigued. She learned about the Back Bay's block captains this summer and plans on attending one of their upcoming meetings to find out more.

''I am very impressed," she said, ''because at the end of the day, it's kind of like peer pressure, isn't it?"