Nuns at Marymount College have taken a vow of poverty, but lay faculty members at the Roman Catholic women's school in Arlington are determined not to do the same.

In the first protest demonstration anyone could remember at the 800-student campus, about 20 teachers and members of their families picketed religious services yesterday demanding higher salaries.

Dr. Boyd Hagy carried a sign bearing the words "Rerum Novarum," the title of an encyclical issued nearly a century ago by Pope Leo XIII. In the document, whose title 'Of New Things," the pope told employers it is their moral duty to pay their workers just wages.

The demostration, which follows more than four months of negotiations for a new contract, was met by an equally adamant counter-demonstration staged by the school's dean and a member of Marymount's board of directors. The board member, Sister deSales, in the blue habit of the religious Order of the Sacred Heart of Mary, planted herself firmly in the path of the pickets, who stepped quickly around her.

"They'll be better off to take what they're getting then to make the school fold up," said Sister deSales, while Dean Alice Mandanis held an umbrella to protect the fiesty nun from a steady drizzle.

At issue at Marymount, as at a many small private colleges across the country, is a financial squeeze born of rising costs and declining enrollments. cAdministration officials say their most recent salary offer, a phased plan that would provide increases ranging from 7.6 percent to 10 percent by the end of 1981, is the highest they can make without endangering the financial future of the college.

Members of the Marymount College Faculty Association, the faculty bargaining unit, claim the difference between that offer and faculty demands is only $8,800.

"If any college can go under for that amount of money, then it probably should," said Rosemary Hubbard, a science professor.

Bundled up against the chill of the morning rain, Hubbard said she and her colleagues feel "professionally demoralized" by the low salaries at Marymount. The average base salary for the school's 44 full-time members is $12,500, according to the association.

The union said all members hold advanced degrees, and 40 percent hold doctorates.

The faculty has been working without a contract since the beginning of Septemer. Although there are no plans for a strike, faculty members said the administration has not bargained fairly.

Dean Mandanis insisted Marymount's latest offer is "not bad" in relation to similar colleges.

"Teaching is a depressed profession," she said. "No small, private, single-sex school can afford to pay as much as, for example, as a large public school system."

None of the six nuns teaching at Marymount joined the faculty protest. Several sisters came out to lend support to Sister deSales' efforts. Sister Benedict, a retired faculty member, called the picketing disgraceful.

"I can't see a devoted faculty doing what this faculty is doing," she sniffed, then dodged back inside to escape the rain.