Betsey Brown, a Lucketts area civic activist, handily defeated retired federal worker Joseph W. Keating yesterday to win the Democratic nomination for the Catoctin District seat on the Loudoun County Board of Supervisors.

Brown took 78 percent of the vote to 22 percent for Keating, and will face incumbent Frank I. Lambert, a Republican, in the general election Nov. 3.

The Catoctin District is among the most rural in Loudoun, stretching from Leesburg north to the Potomac River and west to the Blue Ridge Mountains and the West Virginia border. Catoctin is home to a sometimes contentious mix of working farmers, wealthy estate owners and affluent professionals who commute to jobs outside the county.

Brown and Keating both painted Lambert, a 57-year-old businessman in his first term, as a booster of excessive development. They sparred over who had the better chance of implementing their shared slow-growth agenda by defeating the incumbent.

Brown, 57, a one-time family therapist who returned to her family farm in 1985 after a 30-year absence, has been active in land preservation issues, and was a key member of the Friends of Rte. 15, a group objecting to proposals to build an outer beltway along the north-south highway through Loudoun.

Keating, 50, is president of the Preservation Society of Loudoun and lives near Waterford.

Catoctin was the only one of Loudoun's eight magisterial districts holding a primary yesterday.