Frank Sartwell, 51, the editor of Defenders, the magazine of the Defenders of Wildlife organization, and a lifelong resident of the Washington area, died Friday at his home in Hyattsville. He had emphysema. w

Mr. Sartwell was born in Washington. His father, the late Franklin G. Sartwell, was an editor of the old Washington Times newspaper. The younger Mr. Sartwell graduated from Catholic University and began his own newspaper career on The Washington Star in 1950.

He was a reporter on The Star for five years and then joined the National Geographic Society. He worked first as assistant editor of the National Geographic School Bulletin and then became a staff writer for the National Geographic Magazine. He was the National Geographic's space reporter and covered the 1962 Ranger Spacecraft space shots to the moon and the Mariner II probes of Venus, among other stories.

Mr. Sartwell left the National Geographic in 1969 and for the next two years was managing editor of Science News. He was a free-lance writer for several years, contributing stories to The Washington Post and to the Smithsonian, Washingtonian, Holiday, Parade and other magazines.

In 1976, he was named editor of the Defenders and held that position until his death.

Mr. Sartwell was a member of the Audubon Society and the Nature Conservancy. He also was a photographer.

Survivors include his wife, Marcia, of Hyattsville, two sons, Crispin and Adam, both of Washington, a stepson, Daniel Hovey of Hyattsville, and his mother, Margaret Benit Sartwell of Washington.