Activists from Mosby Woods, a subdivision that straddles the Fairfax city-county line, won their first battle for unification last week when the city council voted 5-1 for a boundary change that would place the whole subdivision under city jurisdiction.

Under the present borders, two-thirds of the neighborhoods's 344 houses are outside the city, and 30 of the homesites are on the city-county line. County residents are bordered by the city on one side and Route 66 on the other, a situation that has led to charges that the county-area households are "segregated" from the rest of the county and have inferior fire and police protection.

Some residents have fought since 1972 for the boundary change, which they say will allow Mosby Woods to become a real community.

Opposition to the move had a financial basis, and came from city administrators who estimated the cost of annexing the additional residential area at about $354,000. Some members of the public and several city officials questioned the validity of that cost estimate and, in spite of some increased expenses, the city council decided to take in the split community.

The ball is now in the county supervisors' court, where little opposition to the boundary change is expected.