Frank Joslyn Baum (who at times used the alias L. Frank Baum, Jr. (3 December 1883 – 2 December 1958) was the eldest son of L. Frank Baum, and the one of Baum's four sons who pursued his father's imaginative legacy most vigorously. His efforts in the realm of Oz, however, brought him controversy and estrangement from his relations.

Frank J. Baum trained as a lawyer, and had experience as a soldier, in the Philippines in 1904 and in World War I. He rose to the rank of lieutenant colonel in the U.S. Army. Baum worked briefly for Reilly & Britton, his father's publisher. He spent most of his adult life in various business ventures.

F. J. Baum's connections with Oz are manifold:

  • He also was credited for writing the 8-minute animated version of the story that appeared in 1933.
  • He wrote The Laughing Dragon of Oz, a 1934 novel that presented a new Oz story. It was issued as a "Big Little Book" by the publisher Whitman. The project involved him in legal difficulties; he was sued by his mother, Maud Gage Baum. Whitman was sued by Reilly & Lee, and agreed not to reprint the book or release more Oz work. F. J. Baum's career as an Oz writer was frustrated.
  • He crafted an adaptation of his father's never-produced play The Girl from Oz, which similarly was never produced.
  • Finally, F. J. Baum collaborated with Russell P. MacFall on the 1961 Baum biography To Please a Child. The book was published after F. J. Baum's death, and has been criticized for inaccuracies and for mythologizing its subject.

An article titled "Corresponding with Frank Joslyn Baum" appeared in the Winter 1982 issue of The Baum Bugle.

F. J. Baum's first wife was the former Helen Louise Snow; they married in 1906 and were the parents of Joslyn Stanton and Frank Alden Baum. Frank and Helen divorced in 1921, after a long separation. Frank Baum married his second wife, Rosine Agnes Shafer Brubeck, in 1932; she died in 1934. He married his third wife, Margaret Elizabeth Ligon Turner (1907–1971), in 1940, and remained married to her until his death. His grandson Roger S. Baum also became an Oz author.

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