The two burly teen-age boys standing on the bridge in Rock Creek Park watched the woman jogger as she ran toward them up the winding trail. The runner, a Washington lawyer who lives on Capitol Hill, knew she had a choice that warm spring day: keep running and pass by the youths or turn around and avoid them. The idea of retracing her steps made her feel self-conscious and overly suspicious; she decided to keep running.

"I anticipated it, so when they grabbed me, I screamed," the woman said. "I think I startled them." Shaking out of their grasp, she sped away down the path.

When she is asked about the incident now, she finds it difficult to articulate the fear she felt. She no longer runs in Rock Creek Park, preferring the Mall and its open spaces. She runs there now with an emergency alarm device, and with her husband or her large dog by her side.

There are no firm statistics on the number of assaults against joggers in the Washington area.

Several local running groups say that there is increased violence against runners, while local police officials say they have not noticed an increase, but acknowledge that many incidents may go unreported.

"We don't see it as that big a problem," said Capt. Robert Hines, a spokesman for the U.S. Park Police. "We feel it's fairly safe" in the parks.

However, the running groups say that violence against runners has altered the daily routines of many local jogging enthusiasts, and robbed some of them of the sense of privacy and escape that is an important part of the activity's appeal.

The most recent serious attack occurred at 6:15 a.m. on June 5, when a 25-year-old woman jogging on the C&O Canal towpath was dragged into a patch of woods, where she was forced to commit sodomy and left tied to a tree. Park Police have made no arrests in the attack.

Although few of the assaults against women runners result in such violence--most of them involve women being fondled or pushed to the ground--the number of incidents of all degrees has local runners frightened.

"You're vulnerable regardless of the time of day," said Susan Cooper, president of the 1,700-member D.C. Roadrunners Club. "You're an easy target. You're female and you're alone."

"It's something you're aware of all the time," said Judy Billings, a Washington secretary who lives in Fairfax County. An avid runner, she jogs downtown in the middle of the day and carries a pressurized air blast alarm for protection. "It's really scary. There's no rhyme or reason. Paranoia just rises up."

"There's no doubt the number of assaults is growing" locally and nationwide, said Liz Elliott, executive director of the American Running and Fitness Association, a nonprofit educational group. The main reason for the increase is that more women are running nowadays, she said. Of the 35 million runners in the United States, about 12 million are women, Elliott said. Only one-fifth that number were jogging five years ago, she said.

The Park Police patrol the area's most popular running trails, including those on the Mall and in Rock Creek Park, the C&O Canal towpath, and along the George Washington Parkway. Last year, they received only 12 reports of assaults on park grounds, Hines said, and have seven reports so far this year. But, he said, "We're probably not getting all the reports," an opinion echoed by officials with other local police agencies.

Heads of local running groups agree, and they are convinced that there have been many more assaults--including cases of women being fondled, grabbed, pushed to the ground and sexually assaulted--than have been reported to police.

"There are a whole lot of incidents out there that the police don't know about," said Sara Fein, a sociologist and runner who conducted a survey of local women runners. The survey, of 100 members of Runhers Unlmited, an organization of local women joggers, found that only 40 percent of those who had been been assaulted or harassed in the last few years filed reports with the police, Fein said.

Of the 100 women polled, 46 reported incidents that ranged from having been fondled to having been assaulted while they were running and 11 said they had been threatened twice, for a total of 57 incidents.

Sixteen of the 100 reported what Fein called "more serious incidents"-- being chased on foot by men or grabbed. About half the attacks took place in parks or on hiking trails; few of them occurred in residential neighborhoods or on running tracks. Fifty-eight percent of the incidents were in daylight, Fein said, including 10 of the 16 more serious incidents.

In light of the increased concern among area enthusiasts, Runhers is planning a women's running festival for this fall in Washington, to be called "Women Take Back the Path," designed to inform runners about the dangers they face while jogging.

Local runners' concern increased last September after the murder of Gail Orstein, 38, a budget analyst for a Washington engineering firm. Ornstein, who lived in the 2400 block of 20th St. NW, was found beaten, strangled and sexually assaulted on a Rock Creek Park jogging path under the William H. Taft Bridge.

Ornstein, who rarely ventured outside her apartment alone after dark, jogged frequently in the early morning, friends said. She was attacked during a run sometime before 7 a.m. that Saturday morning, police said.

"She didn't fear anybody," said Dwight Vasey, the D.C. detective who has amassed a foot-thick file on her case. "That was the problem."

In his investigation, Vasey spent several nights in Rock Creek Park searching for clues. Between 3 a.m. and 4 a.m. one morning, Vasey said, he found 11 men wandering in the woods, or living there. "The woods are alive," Vasey said.

A month after Ornstein's murder, a 32-year-old librarian from Beltsville was found stabbed to death near a jogger's path in Northwest Branch Park, in Montgomery County. Nancy Elizabeth Finch was found nude, and had been sexually molested. Finch had gone out jogging with her dog at 7 p.m., police said, an hour at which she often ran.

Robert Andrew Goodman, a Silver Spring sheet metal worker, has been charged with rape and first degree murder in the case, and is scheduled to go on trial next month.

Although the majority of attacks have been on women, running group members said, some men have also been assaulted.

Bill Norton, 40, an Internal Revenue Service employe who lives in Reston, said he has been attacked twice. The first time was in December while running on a wooded pathway in Reston, when he was hit in the thigh with a pellet from a air gun that left an ugly wound for several weeks. He never found out who shot him.

The second incident took place in March, when Norton said he was harassed and assaulted by the driver of a carload of teen-agers.

Psychologists have advanced a number of theories on why some people go after runners.

"The behavior appears to grow in part out of the resentment people feel at the sense of freedom runners seem to have," said Martin Reiser, a psychologist who is director of behavioral science services for the Los Angeles Police Department. "They're jogging along apparently in a carefree state, with no responsibilities."

Women joggers wearing running shorts may appear to some men to be flaunting their sexuality, Reiser said. "Some men may feel it's a deliberate temptation or provocation," he said. "They feel it gives them license to approach them."

Several women interviewed could not imagine being the object of sexual fantasies when they're hot and sweaty in the middle of their run. Others said news of the assaults has made them feel self-conscious about dressing lightly for a summer run.

Police and running enthusiasts suggested a number of ways women runners might avoid trouble:

* Don't run alone, and vary the daily running route. Police and members of local running groups say that some of the people who attack women runners hide in woods or other covered areas around popular jogging trails and watch the runners, sometimes returning at the same time each day to watch specific joggers.

* Avoid running early in the morning, late in the evening or any time that there are few people around.

* Wear clothes that reveal as little as possible.

* Pay attention. "Look at everybody," said Henley Roughton, a longtime runner and member of Runhers. "Don't hesitate to cross the street or move away from somebody if you have any suspicions about them."

Running organizations also warn runners to avoid the "runner's high" that many joggers fall into as they run, oblivious to the world around them.

Police and running groups say that among the jogging routes with the highest rate of incidents are trails in Rock Creek Park, especially between Virginia Avenue and the National Zoo; the towpath along the C&O Canal, and along the George Washington Parkway between Alexandria and Mount Vernon.

Among the safest areas in which to run are the Mall in daylight, the Hains Point area, across Memorial and 14th Street bridges and in the area of the Pentagon, runners said. In general, residential neighborhoods and running tracks are safe, according to the Runhers group.

While many women joggers are exercising a newfound vigilance in light of the rise in incidents, there are those who resent the need to do so and the lost peace of mind it has meant. One woman jogger from Georgetown said she ran every morning for years on the C&O Canal towpath, but stopped when she heard about the recent sexual assault there. "It's just too creepy now," she says.

Instead she runs on city streets. It's "not as nice or as peaceful," the woman said. "It's infuriating. It's my own private time in the morning."