The purposes of this thesis are to present a study of Jose S. Alvarez, known as Fray Mocho, and his works with particular attention paid to Memorias de un vigilante; to make a study of lunfardo, the slang of Buenos Aires - its origin, its nature, and its influence on language and literature; and to show the important place of Fray Mocho in the history of lunfardo. To accomplish these purposes, in Chapter I, I have undertaken to present a brief sketch of the life of Fray Mocho. Then I attempt to make a brief introduction to his literary work and place it into the literary history of Argentina. Chapter II contains first a discussion of the nature of and importance of Memorias de un vigilante and then a linguistic study, with definitions and etymologies, of the lunfardo vocabulary Fray Mocho gives in "Mundo lunfardo". This is a link with Chapter III which is a study of lunfardo and its historical and sociological background as well as a discussion of the sources of its vocabulary, and the nature of the language. This study of lunfardo closes with a discussion of the influence of lunfardo has had on literature and the spoken language. The conclusion is an attempt to bring together Fray Mocho and lunfardo and show his important place in the history of slang. / Arts, Faculty of / French, Hispanic, and Italian Studies, Department of / Graduate