The Fairfax County Board of Supervisors yesterday selected Warren E. Isman, fire chief in neighboring Montgomery County, to head its Fire and Rescue Department starting July 5. Isman agreed to a pay cut in order to make the move.

Isman, chief of the Montgomery County Department of Fire and Rescue Services since 1977, will take over the same duties in the Fairfax County department at an annual salary of $55,802, $5,301 less than he is now paid by Montgomery.

He said in an interview he has "reached a plateau" in Montgomery County. "I need some challenges. I feel I will get them in Fairfax."

He also noted that Fairfax employs about 950 paid firefighters compared to Montgomery's 700-member paid firefighting staff.

Both departments also have volunteer firefighters--about 600 in Montgomery and 400 in Fairfax.

Isman, 45, declined to specify what challenges he anticipates in Fairfax when he replaces Chief George Alexander, who retired in December after 25 years with the department.

Isman enters a department that has undergone more than a year of interdepartmental reorganization and controversy.

During the past 18 months the Board of Supervisors has implemented several major changes designed to upgrade the department's training programs and integrate volunteer and paid professional firefighters who frequently have been at odds.

The board has ordered two major staff reorganizations in the past year alone. Many of the changes were recommended by a consultant's study commissioned by the board two years ago.

Fairfax County officials said Isman was selected partly because of his success in upgrading Montgomery County's training and paramedic programs and his work in implementing a computerized communications system. Fairfax is developing a similar computerized dispatch system, officials said.

"He's very strong in almost every area," said Board Chairman John F. Herrity, who said Isman was selected from among 300 applicants. "His only weakness is that he was never a firefighter--he came up through the ranks administratively." Isman joined the Montgomery County Department in 1972 as captain in charge of training after working a decade as a publications engineer and manager. He was named assistant fire chief in 1975 and was promoted to fire chief two years later.