A 24-year-old Fairfax County man was arrested last night and charged with murder in the May 7 slaying of Robert H. Land, a 72-year-old former Library of Congress official who was found strangled in his Old Town Alexandria condominium.

Police identified the suspect as Jeffrey Todd Dyson of 2729 Fairhaven Ave. in the Huntington section of Fairfax. Alexandria police assisted by officers from Fairfax took Dyson into custody around 9:30 p.m. nearWashington's Grist Mill, a park area near Mount Vernon Memorial Parkway and Richmond Highway.

Dyson, who lives about two miles from Land's home just across the Alexandria-Fairfax line, was being held last night at the Alexandria Correctional Center pending court arraignment.

Police sources said that detectives investigating the slaying had attempted several times yesterday to apprehend Dyson. Last evening, they received a tip about his whereabouts and spotted his car near the Grist Mill, where they arrested him on a warrant charging him with murder in Land's death.

Police officials released few details last night about Dyson or what led investigators to him.

Land, who retired several years ago as a top official in the Library of Congress reference department, was found by his daughter in his condominium at 601 Wilkes St., a well-to-do section of the city.

Friends described Land, a Civil War and colonial history buff, as a quiet and kindly man who tended a small garden nearby and offered friends tomatoes.

His widow, Elizabeth (Bootsie) Land, was one of the first women on the Alexandria School Board, serving in the 1960s and early 1970s.