Diffraction studies were carried out near 4°K on the influence of rate and method of growth upon the perfection and microstructure of 99.998% purity argon crystals. The crystallography of the twinning observed was directly verified to be of the type expected for a fcc crystal. Some idea of the dislocation content of a well‐annealed argon crystal was obtained. It was concluded that it should be possible to produce crystals having a perfection adequate for a variety of quantitative lattice studies.