A Fairfax County firefighter, described as "the best performer" by his battalion chief, filed a $1 million lawsuit against the county yesterday, charging he was denied promotion because officials have branded him a union activist.

Lt. David J. Lyons, a nine-year veteran of the Fairfax County Fire and Rescue Department, said in the lawsuit that a 1985 internal memorandum said he "continues to make a very clear distinction between labor and management and . . . does not seem willing to identify himself as purely management." His superior said in the memo this "would lead to conflict situations."

Lyons' attorney, Stephen P. Halbrook, called the memo "the smoking gun for this case" and said Lyons was denied a promotion to fire captain because of it. He said Lyons has filed a grievance with the County Civil Service Commission over the promotion and he hopes the case will be resolved by that agency and not in court.

In the lawsuit, filed in Fairfax County Circuit Court, Lyons charged that the basis for the remarks by Battalion Chief Douglas W. Casey was Lyons' longtime leadership role in the Fairfax County Professional Firefighters Local 2068, a union to which almost 80 percent of the county's firefighters belong.

Fire department spokeswoman Pam Weiger declined to comment on the suit.

Lyons, a 31-year-old Centreville resident, asked the court to order his promotion to the rank of fire captain with back pay and interest and award him damages totaling $1 million.