An orthorhombic dipyramidal form and an orthorhombic lath‐shaped form of UO3monohydrate were grown hydrothermally. Optical and X‐ray constants were determined for these crystalline modifications. The X‐ray data corroborated previous data reported by Zachariasen for the orthorhombic laths and by Dawsonet al. for the dipyramidal form. An unreported phase transformation was observed to take place in the orthorhombic dipyramidal phase on the application of pressure. Optical and X‐ray examinations of the new phase showed it to be identical to the orthorhombic lath phase. The transformation appeared to fit Buerger's “displacive transformation” classification. Optical observations made on partly transformed dipyramidal crystals lying on their (001) faces showed the two phases, i.e., the dipyramidal phase and the new phase, joined along a (11.l) dipyramid plane. The displacive movement appeared to have taken place in a (110) dipyrami