The Arlington County Board yesterday voted unanimously to impose a selective hiring program, effective immediately, an action that Republican-backed Independents led by Walter L. Frankland, Jr. claimed as a major - and surprising - victory.

Under the new program, County Manager W. Vernon Ford is ordered to reduce the present number of 2,400 county employes through attrition. His first goal is to cut 120 positions by July 1979 and another 120 by July 1980. According to Ford, only one of three vacancies in the government can be filled under the plan.

A hiring slowdown was regarded as the first major test of the new majority of Republican-backed Independents that will assume control of the five-member board Jan. 1. That group consists of Frankland, Dorothy T. Grotos and newly elected board member Stephen H. Detwiler, who was present at yesterday's meeting. Board Chairman John W. Purdy and Vice Chairman Ellen M. Bozman are endorsed by a Democratic coalition.

"This is not a new thing I just dreamed up," said Frankland, who made a similar proposal that the board defeated in September. "The citizens have had their input (in the recent election) and we're going to institute this in January anyhow, but we'd like your cooperation now."

Detwiler, with Frankland's assistance, campaigned almost exclusively on a tax plan under which the county's property tax rate would be reduced. Frankland told the board yesterday that this proposal would mean a $5 million revenue loss that could be offset by a reduction in personnel through attrition.

"I voted for a selective hiring policy, not a freeze," explained Purdy after the meeting. However, midway through the board's hour-long discussion, Purdy turned to Frankland and said angrily "to expect the present majority to support your motion is unrealistic. I will not vote for any motion that has the tenor or purpose of a freeze."

After the meeting Purdy explained his vote. "There's a lot more leeway here than with a freeze. Besides it's going to happen anyway in January."

"Of course it's the same thing as a freeze," Frankland retorted. "From a political standpoint there was nothing else they could do" but support it. tr for add third

Frankland agreed to withdraw a proposal for a selective hiring freeze when Ford warned him that such wording could jeopardize Arlington's participation in federal programs. "To go on record as having a hiring freeze without a budgetary problem is not desirable," he told the board. "Let's not open ourselves up to a contest with the feds."

Arlington is the third local jurisdiction to impose some sort of hiring program this month. Two days after the November election, outgoing Montgomery County executive James P. Gleason imposed an immediate hiring freeze to help offset a projected $21 million deficit. Last week the Alexandria City Council passed a resolution banning the hiring of additional employes without specific authorization by the council.