McDonald's may have lowered the price of the Big Mac to 55 cents, but that won't affect too many people in Harvard Square, where fast-food chains are about as common as aquatic life in the Charles River.

The Harvard Square Defense Fund has been one of the most effective and pugnacious community groups in Cambridge, fighting to defend the small-town, Mom-and-Pop feel of the Square against what it sees as the encroachment of big-name businesses--the so-called "chain creep."

But hopeful entrepreneurs and real-estate moguls assert that the Defense Fund is a legalistic group of obstructionists whose only achievement has been to stymie free enterprise in the city and usurp what should be the prerogative of consumers.

The 450-member group is adept at winning compromises in development proposals through skillful negotiation--and litigation.

Since 1988, the Fund has filed five lawsuits. In three of those cases, the Fund's tenacity brought results.

* In 1989, the Fund sued Harvard and a developer, Carpenter and Co., after the city Planning Board granted them a permit to convert what is now the Harvard Square Hotel into a six-story office building. Eventually, the University dropped its plans to develop. The developer sued the Defense Fund, but the case was settled out of court, according to Defense Fund President Gladys "Pebble" Gifford.

* In 1992, the Fund sued Harvard, challenging the plans to build Harvard-Radcliffe Hillel's Rosovsky Hall, which required a special permit to provide fewer than the required number of parking spaces. Harvard and the Fund settled out of court, with the University making concessions in the building's height, although that had not been an issue in the lawsuit.

* In 1996, the Fund sued Baldini's, an Italian restaurant chain, contesting the transfer of a special permit allowing the company to open a fast-food restaurant on the Harvard Student Agencies property on Mt. Auburn Street. During the suit, Baldini's dropped its effort, though the Defense Fund maintains the two events were unconnected.

In two other cases the Fund was not so successful.

* In 1989 the Fund lost a major effort to scale down what is now One Brattle Square--home of such shops as HMV Records and Compagnie Internationale Express. In a major defeat for the Fund, the court ruled that it had no legal standing to challenge the project.

* Last year, the Fund's attempt to prevent the opening of a Starbucks on Church Street was thwarted when its suit was dismissed from court.

Detractors say the Defense Fund's tactics are heavy handed--last year it hired a high-school student to photograph people entering and leaving Starbucks Coffee to demonstrate "congestion." But supporters assert aggressive methods are necessary to get results.

Gifford says that "80 percent of the work we do has nothing to do with lawsuits," but tax returns filed by the group with the Massachusetts Attorney General's Office between 1990 and 1995 indicate that 85 percent of the more than $114,000 it raised during that period went to attorneys.

"The Defense Fund plays a necessary watchdog function in the Square," says Charles M. Sullivan, executive director of the Cambridge Historical Commission and a 32-year resident of the city. "It's an area where the stakes are highest and the controversy's the sharpest. And the Defense Fund has been correspondingly vigorous."

Others assert the only result of that vigor has been to leave Square businesses and residents frustrated. "Some people feel that it's a waste of time, energy and money and that the whole process is frivolous," says a board member of the Harvard Square Business Association who asked to remain anonymous.

Losing the Village Feel