Microstructured crystalline optical fiber from silver halides is described. Both experimental and theoretical evidences are presented to establish that the fiber is effectively single mode at wavelength $10.6\mu \mathrm{m}$ with numerical aperture $\mathrm{NA}=0.16$ and optical losses of $\sim 2\mathrm{dB}∕\mathrm{m}$ . Crystalline microstructured optical fibers offer key advantages over step-index optical fibers from silver halide crystals. The wide transmission range of wavelengths $2–20\mu \mathrm{m}$ provides great potential for applications in spectroscopy and for the development of a range of new crystalline-based nonlinear optical fibers.