George B. Hartzog Jr., whose political skills as director of the National Park Service in the 1960s and early ’70s led to the addition of nearly 50 million acres to the park system, more than doubling its size, died on June 27 in Arlington, Va. He was 88 and lived in McLean, Va.

The cause was kidney disease, his wife, Helen, said.

In his nine years as parks director, Mr. Hartzog was attuned to President Lyndon B. Johnson’s Great Society aspirations and the environmental advocacy of Stewart L. Udall, then secretary of the interior. But Mr. Hartzog ruffled feathers during President Richard M. Nixon’s administration, and was fired when a Nixon friend was slighted by a parks official.

During his tenure Mr. Hartzog oversaw the acquisition of 72 sites, amounting to 2.7 million acres. The list went beyond national parks to include recreation areas, seashores, river ways and historical monuments.

“He was an empire builder,” Robert M. Utley, a former Park Service historian, said in an interview on Tuesday. Besides Stephen T. Mather and Horace M. Albright, the agency’s founders, Mr. Utley added, “I judge George Hartzog the greatest director in the history of the service.”