The knowledge which has been acquired as the result of studies by means of polarized light and x-rays of the behavior of rubber when stretched has led to the conclusion that rubber is a polyphase system, in which crystals and the liquid phase are in a state of equilibrium which depends not only on the temperature but to a considerable extent on the pressure. This concept is founded on the observation that there is an increase in the crystalline phase after rubber which has been stretched is allowed to rest, and that the anisotropy shows a reversible effect at definite temperatures and degrees of elongation. Experimental proof of this dependence on pressure has, however, been lacking heretofore, because investigations have been confined to unidirectional stretching, and consequently to pressure in one definite direction. If the conclusion stated above were correct, it would be expected that pressure from all directions would lead to crystallization under conditions whereby it would not otherwise occur. An experiment of this kind was planned at the time of the earlier work, has been in progress since then, and has recently been completed. The results are described in the present paper.