Spray paint the weapon in this battle

The cussing usually starts about a mile beyond the missed turn, when the cyclist figures out he's lost and a bike route marker on the pavement has been painted over. The ''marker Nazi" has struck again.

Wielding a can of black spray paint and an undefined grudge, this mysterious overseer of the asphalt, believed by many cyclists to be a woman, has quietly become the nemesis for thousands of cyclists who depend on the white-and-yellow hash marks to keep them on course along the bike routes that meander through the suburbs north and west of Boston.

No one seems to know who this vandal is, where she will strike next, or why she does what she does. But speculation has become a bit of a parlor game among the two-wheeled set.

''My guess," said Harry Manasewich, a cyclist from Arlington who has ridden with the Charles River Wheelmen for 18 years, ''it's one of the local residents who doesn't like the bikers coming through their neighborhood."

''Nah," said his friend Tom Whitehall, a fellow rider from Westford. ''It's some driver who's [angry] because they had to slow down somewhere and figured 'This will fix their wagon.' "

Whatever the motivation of the marker Nazi -- as she has come to be known -- her actions highlight the growing frustration between the ever-increasing number of recreational cyclists who pedal through Boston's suburbs and the motorists who compete for space on the roads alongside them.

To some drivers, annoyed by the thousands of additional bikers flooding the roads around Boston in recent years, her methods strike a tone of righteous vindication. Even cyclists minding the rules of the road inevitably slow traffic. And the ones who pedal recklessly can terrify drivers forced to swerve around them.

But to cyclists who already feel vulnerable next to 2 tons of rolling steel, the marker Nazi is exacting a prickly revenge and making an already dangerous hobby worse.

Theories on who she is abound, but sightings of an older woman with a can of spray paint have fueled speculation for years, especially around Concord, Lincoln, Bedford, and Carlisle, where her handiwork is most frequent.

''Carlisle is the worst," said Paul Hardin of Lowell, who leads a Friday night ride for the Charles River Wheelmen, one of the largest bike clubs in the state. Hardin is responsible for marking a 28-mile route that starts in Bedford and winds through Concord, Carlisle, Acton, and Chelmsford. ''It happens at least once a year."

Fueling the tensions between cyclists and drivers are achy joints and simple demography. Baby boomers with worn-out knees are hanging up their running and tennis shoes and picking up their old two-wheelers in growing numbers in an effort to stay fit without battering their bodies. Nearly every weekend, May through September, there are hundreds of charity bike rides, circuit races, triathlons, and club rides.

Since 1999, when Lance Armstrong won his first Tour de France, sales of road bikes have climbed about 50 percent nationwide, according to industry experts. Meanwhile, the percentage of cyclists commuting into Boston has risen 50 percent in the last 20 years, according to the Massachusetts Bicycle Coalition. Boston now has the 11th highest percentage of bike commuters in the country, according to a Portland (Ore.) State University study.

With all those additional miles pedaled have come the inevitable run-ins with cars.

''At least once on just about every long ride there's some driver that has a problem with the fact that I'm riding my bike and he honks when he's right on top of you," said Eric Evans, the president of the Charles River Wheelmen. ''There are some people behind the wheel who simply like to be jerks. You can't seem to really escape it."

Cyclists say the least bike-friendly towns are also the most exclusive communities -- and the ones with the most desirable cycling. Enter the marker Nazi.

First seen seven or eight years ago, she obliterates hash marks at critical intersections, making it hard for bikers unfamiliar with an area to discern which turn to take. For cyclists, it can mean miles of aimless meandering.

Paul Harlow, a past president of the Charles River Wheelmen, thinks the bulk of the spray painting is done by just one woman, but he said she's not alone. In Berlin, Northborough, Boylston, and Billerica, route markers have been concealed by people who don't like cyclists in their neighborhoods or markers in front of their homes.

''One guy in Berlin was just doing it to be miserable," Harlow said. ''He told me, 'If you have the right to paint them, I have the right to paint over them.' "

The hash marks, which are little pictograms pointing in a direction, are usually about a foot long and take a variety of shapes. They have been used for at least three decades to help riders follow routes that sometimes stretch 100 miles. It can take hours, even days, for bike groups to lay them down, and several police departments said they have no problem with them as long as they don't interfere with public safety.

If they are properly laid out, they increase safety because riders never have to take their eyes off the road to consult a map. But as cycling has grown in popularity, the markers have become something of a graphic catastrophe. At some of the more bucolic intersections in Concord and Carlisle, there are as many as a dozen different-shaped arrows directing riders on different routes.

Some speculate the marker Nazi is a neat freak who just can't stand the clutter.

''But I'm not sure what she thinks she's going to accomplish," said Manasewich. ''We'll just get lost and wander around her neighborhood even longer."

Douglas Belkin can be reached at dbelkin@globe.com.