Fairfax County Supervisor Joseph Alexander, one of the two remaining Metro board members who can claim to have been present at the creation of the subway system, resigned from the transit board yesterday in a dispute with his fellow supervisors.

Alexander, who had voted earlier in the day to support an important Metro financing agreement with the federal government, abruptly announced his resignation later at a meeting of the county supervisors.

A 10-year veteran of the Metro board, Alexander said he was frustrated by the supervisors' refusal to support another agreement he helped devise to finance completion of part of the 101-mile system. The Fairfax supervisors have refused to support the agreement because it does not contain a specific commitment from Metro for completion of any subway stations in the county.

Fairfax County is the only member of the Metro system which has yet to sign the agreement. Unless all Metro jurisdictions approve it by the end of this month, officials say the system could lost $275 million in federal aid.

After Alexander walked out, the supervisors voted not to accept his resignation from Metro and then went into closed session to assess the action, which could have repercussions in the current election campaigns of six board incumbents.

Instead of supporting the so-called interim Metro agreement, the supervisors said they wanted to meet with the city of Falls Church and perhaps the Metro staff to tell why the county opposes building the West Falls Church station, but not opening it until other stations, farther west on the line at Dunn Loring and Vienna, are also built and in operations.

Falls Church had threatened to tie up the agreement, and cause the loss of federal funds, if the West Fall Church station became a temporary terminal. The city said that it could not handle all the congestion that whould be created around the station which is located on the Falls Church-Fairfax County line.

If Alexander's resignation becomes final, Cleatus Barnett of Montgomery County would be the only member of the Metro Board who was serving when the plans for the subway were approved.