Disks from a number of igneous rock specimens have been cooled from temperatures above the Curie points of their magnetic minerals under directed stresses of 500 and 1000 kg/cm2. In all cases the thermoremanent magnetizations (TRM) measured after unloading coincided with the direction of the earth's field (0.6 oersted) to which they were exposed during cooling, and which was either at 66° or 46° to the axis of compression. A duplicate set of control specimens was given the same heat treatment but without the application of stress and found to be magnetized in the same direction. The scatter of measured directions of magnetization was the same for both sets of specimens, showing that any magnetostrictive deflections of magnetization must have been less than the experimental error, about 3°. Another set of specimens was given isothermal remanent magnetization (IRM) by exposure to a field of 140 oersteds at an angle of 49° to the axis of directed stress. These specimens as well as a duplicate set of unstressed control specimens were also found to be magnetized in the direction of the inducing field. The experiments show that the application of directed stresses up to 1000 kg/cm2 during the magnetization of igneous rocks containing a variety of magnetic mineral aggregates does not deflect their magnetic moments. The same result was obtained with a rock in which the magnetic mineral was changed by heating in air while under directed stress. It is concluded that the natural magnetic moments of isotropic igneous rocks are not normally deflected by magnetostrictive effects.