Crystals of thallium-bromo-iodide, or KRS-5, are transparent to radiation of wavelengths between 0 5 μ and about 40 μ This note briefly describes the growing of KRS-5 single crystals and their optical working The crystals are grown in vertical cylindrical crucibles by a progressive freezing of the melt from bottom to top The physical, chemical and mechanical properties of the material are summarized