The 758-member white-collar work force of the Washington Suburban Sanitary Commision voted narrowly yesterday to unionize as a local of the American Federation of State, County, and Municipal Employes.

Yesterday's vote represented a important victory for AFSCME, which last month led Prince George's 1,500 public-employe union out on an unsuccessful 11-day strike.

The WSSC provides water and sewer service in Montgomery and Prince George's counties.

To win yesterday, the union needed support from 45 percent of the agency's white-collar work force, which includes clercal employes, engineering assistants and inspectors. Forty-nine percent of the group -- 369 workers -- voted for the union, according to an unofficial count.

AFSCME, which already represents about 600 blue-collar WSSC employes, will form a separate local for the newly organized group, according to union business agent Paul manner. He said he expects negotiations for the local's first contract to start in three to four weeks.

The WSSC did nothing to combat the AFSCME's organizing drive, according to union and agency officials.

"We've been keeping our hands off," said WSSC Assistant Personnel Director Herbert Butler. "We've got a neutral attitude toward the whole thing."

Manner said he expects yesterday's vote to give AFSCME "renewed credibility" in the aftermath of the Prince George's strike.

"Everybody was looking at the WSSC campaign to see if there would be any kind of backlash," he said. "I was concerned myself."