Bohmite, gamma-AlOOH, has been found to exist in nature and may be prepared in the laboratory in a variety of ways. From the close similarity of the powder x-radiograms of bohmite and the mineral lepidocrocite, gamma-FeOOH, it has been concluded that the structures are analogous. The existence of the closely related compound gamma-ScOOH has clearly been established by x-ray diffraction and dehydration isobaric studies, but quantitative x-ray studies have not been made. There are two crystalline modifications of the monohydrate of aluminum oxide with the formula AlOOH. The modification corresponding to diaspore, alpha-AlOOH, has been found to be isomorphous with goethite,alpha-FeOOH, of known structure. The second modification corresponding to bohmite, gamma-AlOOH, is isomorphous with lepidocrocite, gamma-FeOOH. The structure of lepidocrocite has been determined by Ewing employing oscillation and rotation photographs of a naturally occurring single crystal. Even though bohmite occurs in nature, it has not been possible to obtain individual crystals suitable for single crystal x-ray studies. Milligan and Weiser, from dehydration isobars of scandium trihydroxide and x-ray diffraction studies, reported the existence of scandium oxide monohydrate, gamma-ScOOH, which they believed to be isomorphous with bohmite. In the present investigation the unit structure, space group symmetry, and detailed atomic arrangement of gamma-AlOOH and gamma-ScOOH have been determined. Since numerous attempts to grow single crystals of both of these monohydrates failed, it was necessary to confine the work done here to powder techniques.