FIELD: terahertz materials. SUBSTANCE: invention relates to terahertz (THz) materials, specifically to terahertz silver halide light guides for the high-frequency range from 10 to 30 THz, intended for medical, laser technologies and new generation THz equipment. According to the invention, a terahertz silver halide fiber of the AgCl_0.25Br_0.75 - AgI system is proposed, including a solid solution of AgCl_0.25Br_0.75 and additionally containing silver iodide in the following ratio of components in mol.%: AgCl_0.25Br_0.75 98.0-84.0; AgI 2.0-16.0. EFFECT: invention solves the problem of creating photo- and radiation-resistant, non-toxic and non-hygroscopic, flexible silver halide optical fibers that are transparent in the high-frequency terahertz range from 10 to 30 THz and in the infrared range from 30 to 10 μm with low optical losses up to 0.3 dB/m. 1 cl, 2 dwg, 3 ex.