This thesis is a critical edition of the Judeo-Arabic commentary on the Hebrew text of the Book of Job by one of the greatest Karaites of his age (second half of the tenth century A. D.), Yefet Ben 'Ali the Karaite. An examination of the photocopies and microfilms of the original Manuscripts of Yefet Ben 'Ali written in the XIth, XIV-XVIIth, XVth and XVIth centuries resulted in a delimitation of the number of chapters in this edition i.e. chapters I-X. None of the four Manuscripts is complete and I have tried to complete the presentation of the first ten chapters of Yefet's commentary on the Book of Job by filling in the gaps of the master copy which I used (Ms. A., Or. 2509 B. M. ) from the other Manuscripts. I used it as a main text because it is almost a complete copy compared with the others, as far as the first ten chapters are concerned. The four Manuscripts which I used are housed in the British Library in London. This edition is prefaced by an introduction, comprising a discussion of the information we possess about Yefet's life in Basrah and Jerusalem, with reference to his works in general and the authenticity of his work on the Book of Job in particular. This is followed by a description and analysis of the commentary, discussing the method used by the commentator, and how he made it possible for large numbers of Jews in non-Arabic speaking countries to make free use of his interpretations of biblical texts allied to the Karaite theological viewpoint and its relationship to Mu'tazilite views. There follows an analysis of the language used by Yefet in his text and exegesis, i.e. morphology, orthography and so on. A comparison is then made with Saadia Gaon, including a brief discussion of the language and exegesis of the two scholars which deals with the fundamental characteristics of Judeo and classical Arabic; in addition, notes on the text are appended in which attention will be drawn to Yefet's characteristic vagueness in interpreting the Hebrew text of the Book of Job. Special attention is paid to the vowels in each of the Manuscripts, and differences between the Manuscripts are footnoted throughout the text of this edition. The appendix takes cognisance of M. E., i.e. Opp. Add. 4.165 of the Bodleian Library, listing fully the differences between it and the printed text of this edition.