LOS ANGELES, FEB. 13 -- In a landmark concession to revolutionary changes in the American family and society, the Boy Scouts of America have dropped their 78-year-old ban on women as troop leaders.

The decision, made by a vote of the Boy Scouts' national executive board Thursday in Washington but not immediately announced, was hailed today by women who have been fighting the issue in court and by divorced mothers who have been hampered in encouraging their sons' scouting.

"It is a victory for all scouts from single-parent families," said Phyllis Gibson, a divorced mother in Goleta, Calif., who had to cancel a camping trip for her sons' troop last year because of the men-only rule. "It means they will have the same contact with scouting as families that have a father at home."

A recent favorable court ruling in Gibson's fight against the old rule helped persuade the Boy Scouts to capitulate, her Los Angeles attorney, Gloria Allred, said.

"Troops have been falling down because there aren't enough men to lead them," said Catherine Pollard, a 69-year-old scouting enthusiast in Milford, Conn.

Pollard has been fighting the men-only rule since 1974, when the Boy Scouts denied her request to be formally declared scoutmaster of a troop she was leading. She lost her case in the state Supreme Court last year on a technicality but said today the rule change will allow her to look for a new troop to lead.

Boy Scout executives at the national headquarters in Irving, Tex., have defended the rule, which dates to 1910, as necessary to the special quality of the Boy Scout experience. A letter from chief scout executive Ben H. Love last year spoke of "the principle that developing boys {of that age} need a close association with adult males who can provide models of manhood."

Now, however, United Press International quoted Boy Scout spokesman Barclay Bollas as saying, "the decision is that we have removed all gender restrictions on all volunteer positions in the organization." Girls, he said, will still be barred from membership.

A letter sent from the Irving headquarters to all local scouting councils said: "It is time to recognize that in a changing society the unique strength of our organization lies in the dedicated efforts of both men and women . . . . Our efforts must be focused on obtaining the best possible leadership -- male or female -- to carry forward the work of the scouting movement."

About 500,000 of 1.1 million adult volunteers in the Boy Scouts are women, but they have been restricted to lower-level posts. The new rule specifically opens to women assignments as leaders and assistant leaders of Webelos dens for 10-year-old Cub Scouts; scoutmasters and assistant scoutmasters of Boy Scout troops, whose members are usually 11 to 17; and coaches and assistant coaches for Varsity Scouts, ages 14 to 18.

Last year Boy Scout attorneys asked a Superior Court judge in Santa Barbara, near Gibson's home in Goleta, to dismiss her suit on the grounds that the Boy Scouts is not a business organization and thus not subject to California's civil rights laws. The judge overruled the motion, and Allred said she was planning to seek an injunction allowing Gibson to supervise a camping trip.

Although Gibson never sought to be a scoutmaster, the Boy Scouts insisted that even a temporary substitute in that role, such as an adult supervising a camp out, had to be male. Some fathers with little interest in camping discovered to their chagrin that only they, and not their wives, could be enlisted for overnight duty.

Pollard protested the ban on female scoutmasters in the Connecticut Commission on Human Rights and Opportunities and then in the courts until the state Supreme Court ruled that state antidiscrimination laws did not apply because she was not paid for her Boy Scout work. But her attorney was planning to challenge the rule on other grounds, and "a lot of other women in the country were getting ready to go to court," she said.

Gibson said her sons, Ron, 18, now a junior assistant scout leader, and Chad, 16, "were really excited" about the rule change. She said she expects Troop 129's planned camping trip to Catalina Island this summer to proceed without difficulty.