The mechanical anisotropy of polyethylene has been investigated by making measurements of tensile modulus on thin strips cut at various directions in the plane from anisotropic polyethylene sheets with a wide range of physical structures. Conventional sheets possessing fiber symmetry were made by uniaxial stretching of high- and low-density polyethylene sheets, in some cases accompanied by heat-annealing. Sheets of lower orthorhombic symmetry were made from oriented low-density sheets by rolling and annealing processes recently established by Hay and Keller [1]. In all cases the moduli in directions making angles of 0, 45, and 90° with the initial draw direction were measured over the temperature range −125 to 50°C. The most remarkable feature of the results is that the patterns of anisotropy change with temperature, the relative values of E0, E45, and E90 altering in a manner which produces crossover points. These results generalize the result previously established by Takayanagi et al. [2] that the modulus perpendicular to the draw direction in uniaxially oriented sheets (E90) becomes greater than the modulus parallel to the draw direction (E0) above a characteristic temperature. In the orthorhombic sheets the a-, b-, and c-axes of the crystallites, as revealed by wide-angle X-ray diffraction, take up orientations which are directly related to the macroscopic dimensions of the sheet, the exact correlation depending on the details of preparation. This means that measurements of E0 and E90 for different sheets can give direct measures of Ea, Eb, and Ec, where these moduli now refer to crystallographic directions in the structure. It was found possible to correlate the modulus measurements on two types of orthorhombic sheets to a reasonable approximation. Moreover, the modulus data on the orthorhombic sheets could also be related to the modulus data on sheets of fiber symmetry, provided that the latter had been suitably annealed. Tentative molecular interpretations of the changes in moduli with temperature are attempted. In addition to the influence of branchpoint mobility which appears to result in shear on some crystallographic planes, it is proposed that, in sheets having a well-defined lamellar texture, shear between crystal lamellae can play an important part in determining the mechanical anisotropy. The postulate of interlamellar shear at high temperatures gives a consistent explanation of the modulus crossover points for all samples.