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Rhys Llywelyn Isaac (20 November 1937 in Cape Town, South Africa – 6 October 2010 in Blairgowrie, Victoria, Australia) was a South African-born Australian historian of American history who also worked in the United States.
In 1963 Isaac emigrated to Australia, where he taught at the University of Melbourne, and later at La Trobe University (1971–91), where he was Emeritus Professor of American History. In 1975 he was a Distinguished Visiting Professor of Early American History at the College of William & Mary in Williamsburg, Virginia.
In 2004 Isaac published Landon Carter's Uneasy Kingdom: Revolution and Rebellion on a Virginia Plantation, which made use of the exemplary diary of a Virginian landholder and member of the House of Burgesses.